refactor: remove hoisting and add JSDoc to matchTag in Emmet

[Vedansh Shetti (vedanshshetti)]

Description:

This PR refactors extensions/emmet/src/matchTag.ts to improve code readability and logical flow.

Changes:

• Moved the helper function getUpdatedSelections above the main exported function matchTag. This follows a "define-before-use" pattern, allowing readers to understand the implementation details before seeing the high-level logic.
• Added JSDoc comments to matchTag to clearly describe its purpose and behavior.
• Added explicit : void return type to matchTag for better TypeScript clarity.

Motivation:

The previous structure required jumping to the bottom of the file to understand the core logic used in matchTag. By reordering the functions and adding documentation, the file becomes easier to read and maintain for new contributors without changing any runtime behavior.

Side-Note, if you think the commit message looks off or something, that's because I used GH Copilot to write the commit msg. But I wrote the code myself.

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