fixes a crash in the splitter when the last char is a closing
parentheses.

we'll have to deploy to see if this actually works – using `mike serve`
apparently doesn't deploy the local `docs/` directory, but uses a git
branch.

I wanted to extract the treesitter-context that's used in completions,
so it can also be used for the on-hover feature.

I've also tried to clean up the APIs, detangled completion-context and
sanitization, and extracted the general test helpers into the
`pgt_test_utils` crate.

brings home the custom binding that was experimented in pg_parse.

no functional changes, just different imports and a bit of cleanup.

also removes the libpg_query submodule - we are now cloning the repo
"live" in the build script.

closes #453

adds support for plpgsql_check. the approach is even simpler as
described in the issue: if we encounter a create function statement, we
start a transaction, run the statement, then run plpgsql_check on it,
and then rollback the transaction.

the remaining code is just about translating the location information we
get from the extension into a good range.

todo:
- [x] integrate into workspace api
- [x] to get span, move from the first word / occurrence in the line to
the next semicolon
- [x] handle return trigger by checking for all variations
- [x] make sure we do not report create fn syntax errors if
plpgsql_check is enabled
- [x] check why "not a known variable" is not getting the right span

closes #190