feat: add change selections command

[Myriad-Dreamin]

Adding a command to track user selections in the editor, then we can use the focusing_selection stored in the global state:

tinymist/crates/tinymist/src/server.rs

Lines 79 to 91 in 8db172c

	/// The client focusing selection ranges.
	///
	/// To reduce complexity, we only record the primary selection range, while
	/// there can be multiple ranges selected by the client at the same time.
	/// - Example: In VS Code, you can create multiple cursors by `Ctrl + D`.
	///
	/// To reduce complexity, A further question that we haven't covered
	/// currently is that: The client may select ranges in multiple files at
	/// the same time, but we only preasume that all of the selections are
	/// in the [`Self::focusing`] file.
	/// - Example: In VS Code, you can pick multiple ranges in multiple files by
	/// the text search command.
	pub focusing_selection: Option<LspRange>,

This is unfriendly for the editors that cannot make client-side extensions.

[yassine-safraoui]

I think the doc.isUntitled is a mistake, you probably meant !doc.isUntitled, because, just before on line 166 you return if the document is untitled which makes sense because you don't want to consider it.

Suggested change

	if (doc.isUntitled && window.activeTextEditor?.document === doc) {
	if (!doc.isUntitled && window.activeTextEditor?.document === doc) {

[Myriad-Dreamin]

Hello, I added comments to the code. I checked it and think of I'm not doing a bad refactor. Please check the comments:

tinymist/editors/vscode/src/extension.ts

Lines 208 to 211 in f4614ee

	// Watches the active editor that owning a titled `document` (`!isUntitled`).
	//
	// todo: plaintext detection
	window.onDidChangeActiveTextEditor((editor: TextEditor | undefined) => {

tinymist/editors/vscode/src/extension.ts

Lines 216 to 223 in f4614ee

	// Watches the active editor that owning an untitled `document` (`isUntitled`).
	// `onDidChangeActiveTextEditor` doesn't capture changes of untitled `document`s. This is because when the user
	// change language id from `plaintext` to `typst` manually, the editor is not changed but only replacen with a new
	// document with language id `typst`, in which case vscode doesn't trigger `onDidChangeActiveTextEditor`.
	//
	// FIXME1: we could do better by finding a way to handle the focus of `document` with only one handler.
	// FIXME2: seems like we are also failing to capture changes of language id of titled `document`?
	vscode.workspace.onDidOpenTextDocument((doc: vscode.TextDocument) => {