Support js/wasm build

[ngs]

Summary

This PR makes gocui compile for GOOS=js GOARCH=wasm, so gocui applications can run in a browser using tcell's WebAssembly backend (webfiles/).

Currently a js/wasm build fails because gui_others.go relies on tty ioctls and signals that do not exist on js:

gui_others.go:35:34: undefined: syscall.SIGWINCH
gui_others.go:38:37: undefined: syscall.SYS_IOCTL
gui_others.go:39:30: undefined: syscall.TIOCGWINSZ
gui_others.go:49:16: undefined: syscall.SIGWINCH

Changes

Commit 1 — build support

• Exclude gui_others.go from js builds (//go:build !windows && !js)
• Add gui_js.go with a getTermWindowSize implementation that asks the tcell screen for its size, which is how the WebAssembly backend reports the emulated terminal dimensions

Commit 2 — let the hosting page control the terminal size (optional but makes browser apps practical)

• If the page defines terminalCells() returning [cols, rows], the terminal is sized to it at startup instead of tcell's default 80x24
• A global tcellSetSize(cols, rows) is registered so the page can resize the running terminal from a resize listener; tcell posts an EventResize and gocui relayouts as usual
• The page is treated as an untrusted boundary: thrown exceptions and malformed values are ignored. Screen.SetSize is looked up via an interface assertion because the tcell version pinned in go.mod (v2.4) predates it, so this PR requires no dependency bump; applications building with a newer tcell get the behavior automatically

Testing

• go build ./... and go test ./... pass on darwin
• GOOS=js GOARCH=wasm go build ./... now succeeds
• Verified end to end with a real gocui application compiled to wasm and running in a browser via tcell's webfiles, including dynamic window resizing: https://koikoi.ngs.io (source: https://github.com/ngs/go-koikoi)

[dankox]

It might be worth to just update tcell to newer version where SetSize is available, provide a new API in gocui to enable SetSize and change the NewGui in gui.go on line 138 to:

	g.maxX, g.maxY = screen.Size()

No need for the gui_others.go and gui_windows.go anymore after that change (it's all handled by tcell).

Then in your wasm app, you could just use the gocui.SetSize without needing a JS wrapper to check for specific global functions.

[ngs]

Thanks @dankox, that is a much better shape than what I had. I reworked the branch along your suggestion.

What changed

• NewGui now always takes the size from screen.Size(). With that, the platform specific getTermWindowSize implementations are redundant, so gui_others.go (tty ioctl), gui_windows.go (GetConsoleScreenBufferInfo) and the gui_js.go I had added are all gone. No syscall, no unsafe, no syscall/js in the package any more, and no runtime.GOOS branch in NewGui.
• New Gui.SetSize(x, y int), a thin wrapper over tcell.Screen.SetSize. The new size is picked up by the next flush, which already reads screen.Size(), so nothing else had to change.
• tcell is bumped to v2.9.0. One note on the version: Screen.SetSize actually landed back in v2.6.0, but v2.9.0 is the first release whose wScreen implements the full screenImpl interface (GetClipboard was missing before), so it is the oldest tag that compiles for GOOS=js GOARCH=wasm. Happy to move it to the latest v2.13.x if you prefer.

Two things worth your call

1. Go version. tcell v2.9.0 requires Go 1.23, so the go directive of both modules moves from 1.13 to 1.23.0. That in turn breaks the CI matrix, which was pinned to 1.13/1.15/1.16, so I bumped it to 1.23/1.24/1.25 and added a GOOS=js GOARCH=wasm build and a go test ./... step (the workflow only checked formatting and built the examples). The Go 1.19 style doc comments then trip the gofmt -s job, so there is a separate, purely mechanical commit reformatting them — feel free to drop that one if you would rather handle it apart from this PR.
2. Dropping the ioctl path. gui_others.go existed for FYI: gocui triggers a bug in Go 1.13 (pre-release) #33 (a size of 0x0 inside a docker container, waiting on SIGWINCH until a real size arrives). Modern tcell resolves the size itself and falls back sensibly, so I removed it as you suggested, but flagging it since it is the one behaviour this PR changes for non-wasm users.

Verified

go build ./... for darwin, linux, windows and js/wasm, plus go vet ./... and go test ./..., all green; the _examples module builds too. I also wired the app this came from against this branch and built it for js/wasm: the JS side is now just a resize listener calling into the app, which calls gocui.SetSize — no globals for gocui to discover.

For context on the tcell side: the wasm backend fixes I have been leaning on (gdamore/tcell#1150, gdamore/tcell#1151) are ones I sent upstream and gdamore merged earlier this month, so the browser path is in reasonable shape now.