Support using a different harper-ls executable than the one bundled in the VSCode extension

[mcecode]

Closes #321

I didn't add a test for the new setting since it'd be hard to distinguish if the extension is actually using a different binary or if it's just ignoring the setting and is using what's bundled because output wise, changing to a different executable should yield the same diagnostics in the editor.

[elijah-potter]

Looks great to me! Merging...

As an aside: we're setting up more extensive documentation on the website. Once that's done, we'll want future documentation to be put in there.

Initial drafts of the VSCode page are here.