More robust error handling in the language server

[dfreeman]

This PR addresses a handful of scenarios where the language server wasn't behaving well in actual usage.

• Any unhandled errors thrown within GlintLanguageServer are caught and logged, and a sensible "no answer" response is sent instead
• @glimmer/syntax gives entirely different errors based on whether the syntax problem came from the handlebars parser or the secondary HTML layer. We now no longer choke on the latter type, and make a best-effort attempt to extract the error location in that scenario as well.
• We no longer spew Internal error: offset out of bounds when an LSP request comes in on a template file with syntax errors
• We no longer swallow zero-length transformed spans if the original span was also zero-length (this matters in TS 4.3 because they treat module definitions as an empty span at the start of the document instead of as the entire document like in <= 4.2)

I think this hits all the known crash points we've run into so far, though I'm sure there are others lurking in slightly less obvious corners.