Without this, diagnostics will linger after closing a file.
This refactors towards a pattern of putting outgoing calls as methods on
`Context`. I'm mixed on this, mainly thinking it's an improvement on
using `outgoing` directly (because it shares the name and structure),
might want to move it to a side-class later that is _only_ LSP wrappers.
I could also make inheritance private on OutgoingMessages and these
kinds of methods public there, but I'm hesitant to adopt that approach
versus a type separation.
Add caching of parsed documents, and testing of the textDocument
handlers. This is based on #4896, which splits out some of the
boilerplate to calls.
Note, this caches the entire parse state because we'll want to try to
emit diagnostics when we see the update, without waiting. It may be
helpful to do that asynchronously, but we don't want to wait for another
call (such as documentSymbol). Really, we'll probably want to also add
check for diagnostics, at least.
Jon Ross-Perkins