(feat) add documentation strings to hover response

[fnune]

Fixes #281
Partially fixes sveltejs/svelte#5102 and #280 (still need to produce a docstring for default exports)

For a snippet:

<script>
    /** Documentation string for a */
    const a = true
</script>

Improves the hover contents from:

const a: true

To:

```typescript\nconst a: true\n```\n---\nDocumentation string for a

Which should print out more or less like this:

const a: true

---

Documentation string for a

If there is no documentation, the result is the same as before (for example, just const a: true in a Markdown TypeScript snippet).

Following the documentation here.

[fnune]

I don't know how to set up my editor to use my local version of the LSP so I haven't actually tested this "live".

[dummdidumm]

Thanks for the PR! To test this, first uninstall the extension and then follow the steps documented here

[fnune]

Good news, it's working just fine!

Some samples.

With a simple docstring:

With no docstring:

With a complex docstring:

[dummdidumm]

Great stuff!