Feature: SQLite-for-WASM support

[unhighghlow]

This change adds a new feature sqlite-wasm Detects when the target is WASM, which, when enabled, replaces libsqlite3-sys with sqlite-wasm-rs.
It is a new database driver (in the sense that the sqlite-wasm feature should replace the sqlite feature, unlike sqlite-unbundled)
But it is also not new, since no new APIs were created

Does your PR solve an issue?

fixes #2985

Is this a breaking change?

No, as the new database driver: sqlite-wasm is only activated with the new feature, and without it, no dependencies or APIs are changed. (Except for the fact that some versions in Cargo.lock were modified, which I have no idea how to stop)
Edit: it now uses sqlite-wasm-rs when the target is wasm. This changes the behavior of compiling sqlx with sqlite on WASM from "compilation error" to "success". I don't think this is a breaking change

Problem 1

SQLite-WASM does not support load-extension, which is included in the default configuration for sqlite. There are multiple ways to solve this:

1. Make sqlite-load-extension cause a build error on WASM, but keep it turned on by default
   • Pros:
     • No workarounds
     • No breaking changes
   • Cons:
     • Very inconvenient for users
   1. It is also possible to add a new feature sqlite-wasm, which decreases the inconvenience
2. Make sqlite-load-extension cause a build error on WASM and remove it from the defaults
   • Pros:
     • No workarounds
     • No inconvenience
   • Cons:
     • Breaking change
3. Make sqlite-load-extension cause a build error on WASM, but only when explicitly chosen, with the default being silently ignored
   • Pros:
     • No breaking changes
     • No inconvenience
   • Cons:
     • A janky workaround

Problem 2

The sqlite version used for compile-time checking (the system one) is different from the sqlite version used at runtime (WASM). This isn't actually a big problem, because the same already happens when using sqlite-unbundled

Todo

• Add basic functionality
• Confirm that the APIs of sqlite-wasm-rs and libsqlite3-sys are identical (to be fair I don't really know)
  Oh no

This library is not thread-safe

• Expose sqlite-wasm-rs features
• Add docs
• Add tests

[luveti]

I've been attempting to test this PR out. It believe its crashing when unwrapping the call to SqliteConnectOptions::connect().await, though it's hard to tell exactly where it's coming from.

The error is:
Err(Io(Error { kind: Unsupported, message: "operation not supported on this platform" }))

I believe it may be due to the use of a thread in ConnectionWorker. https://github.com/launchbadge/sqlx/blob/main/sqlx-sqlite/src/connection/worker.rs#L109

It's possible wasm-bindgen-spawn could be used to get around this; But unfortunately won't help in my case, as I'm trying to use this in a service worker, which can't spawn Workers at the moment..