feat: jump from ref to label position

[iamhyc]

closes #261

[Copilot]

Pull request overview

Adds “Go to Definition” support for LaTeX references so users can Ctrl/Cmd+click \ref{...} and jump to the corresponding \label{...} within an Overleaf-Workshop project (closes #261).

Changes:

• Extend ReferenceCompletionProvider to also act as a vscode.DefinitionProvider.
• Parse the reference under the cursor and resolve its target \label{...} by searching open editors first, then project .tex files.
• Register a definition provider for **/*.{tex,txt} under the Overleaf workspace scheme.

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