Feature - Syntax highlighting in the vscode extension

[YuryKL]

Adding functionality from github.com/YuryKL/textmate-datastar to the official extension.

I confirm that I have read the contribution guidelines and will include a clear explanation of the problem, solution, and alternatives for any proposed change in behavior.

Problem Statement

The VS-Code extension provides autocomplete via snippets, but has no syntax highlight for Datastar attributes and expressions making small typos harder to visually identify.

Proposed Solution

Integrate a TextMate grammar into the official extension to provide syntax highlighting in a wide variety of markup files alongside the existing snippet functionality. The highlights include:
The grammar provides color-coded highlighting for:

• Datastar attribute structure (data-[plugin]:[key]__[modifier.arg])
• Datastar expressions (signals with $, actions with @, + JS syntax)

Changes

• Added datastar.injection.tmLanguage.json textmate grammar file
• Added datastar.customAttributes configuration setting to highlight custom attribute plugins
• Added grammar generation/regeneration logic into extension.js to watch for configuration updates

Alternatives Considered

Keep as seperate extensions: maintaining textmate-datastar as a standalone would avoid merging codebases, but users would need to install two extensions that may have different versions.
LSP: This is another option to get the same functionality in VS-Code but is much more complex to build/maintain.

[bencroker]

Thanks so much for the PR! I‘ll review it in the coming days.