astro: Ensure Typescript is present (#14849)

The current Astro Extension fails to load properly if it can't find a
`tsserver.js` file in the current workspaces' `node_modules` folder.
This happens pretty frequently, either if `typescript` is not installed
in the project (which it isn't by default), or if `node_modules` is not
in the workspace root.

This PR adds a fallback method of installing `typescript` alongside the
extensions' language server if it is not found in the workspaces'
`node_modules`, as well as correctly setting the `tsdk` path in the
initialization options.

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>