Adding functionality to render Latex as SVG.

[obousquet]

This introduces a Jsx component which renders a latex string as SVG.
This can be used for example in an expression presenter to display a goal in Latex, or in an HTML display.
See Demos/LeanExpr.lean for usage examples.

[Vtec234]

Thanks for this, and sorry to take so long to review it. I think that having LaTeX is important enough that it should be dealt with upstream. In leanprover/vscode-lean4#534 I introduced a Markdown component which can render math to the @leanprover/infoview package. If that gets merged, all user widgets will be able to make use of it. If not, we can reconsider something ProofWidgets4-specific.