[Feature] add support for Svelte

[kkoomen]

See #628 for the related request.

The idea is that *.svelte files, containing the svelte filetype, will call a custom SvelteParser which will parse top-level script element contents and use the TypescriptParser to parse the inner contents. Svelte therefore automatically extends the same documentation standards (jsdoc for now) as implemented for all the JS/TS filetypes.