fix: define __SOLID_SERVER_COMPONENTS__ for every environment

[ryansolid]

Fixes #396.

Problem

@solidjs/router ships a client fallback for server-component forms. Only server-component apps need it, but every router app pays for the bytes, because a runtime flag does not fold under Rolldown or esbuild. A build-time literal does.

Fix

The plugin always defines __SOLID_SERVER_COMPONENTS__:

• "true" when serverFunctions.components is set, including 'external'; otherwise "false". It is never left undefined, so the dead branch can be eliminated.
• Top-level define, which covers build output and dev source via /@vite/env.
• Every environment's optimizeDeps.rolldownOptions.transform.define. The optimizer ignores top-level define, and Vite only seeds the client environment from top-level optimizeDeps, so configEnvironment sets the flag on each environment. A value already present on that environment is left alone.
• A user-provided define.__SOLID_SERVER_COMPONENTS__ wins, and the optimizers follow it unless they set their own.

Tests

examples/ssr/test/server-components-define.mjs (wired into the example test script):

• Resolved config: off by default, on for components: true and 'external', on both the client and ssr defines and both optimizers. A user define wins. A per-environment optimizer value wins only there. The client scanner JSX runtime stays classic.
• Production build of a fixture: false drops the SC_DEFINE_ON branch, true drops SC_DEFINE_OFF.

Left open

The name and the scoping are the contract this ships. __SOLID_SERVER_COMPONENTS__ is what #396 proposes; core docs (RFC 11 / the server-components skill) and the router guard are follow-ups in other repos, not this PR. The optimizer flag is applied from configEnvironment for every environment, which is wider than the issue sketch's top-level optimizeDeps (that only reaches client). Please confirm the name and that all-environment scoping before merging.

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@solidjs/vite-plugin	Patch

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