`EmptyExpr` is somewhat unique, in that it's constructed in a circumstance
where the parser has been looking for a particular token or string of tokens
and has failed to find any. This means the parser state when constructing
`EmptyExpr` is fairly unique.

This gives rise to a bug where the parser constructs `EmptyExpr` with a
backwards span - a `start` value that's beyond the `end` value. This likely
happens because of the strange state the parser is in when recovering with
`EmptyExpr`.

This commit adds a backstop/workaround to avoid constructing such broken
`EmptyExpr` spans (or any other kind of span). Eventually, the parser state
should be fixed such that this does not occur, but that requires a
significant change to the parser's functionality, so a simple fix in th
interim is in order.

PR Close #41581

…tions (#41581)

This commit changes `getTemplateAtTarget` to be able to identify when a
cursor position is specifically within the argument span of a `MethodCall`
or `SafeMethodCall` with no arguments. If the call had arguments, one of the
argument expressions would be returned instead, but in a call with no
arguments the tightest node _is_ the `MethodCall`. Adding the additional
argument context will allow for functionality that relies on tracking
argument positions, like `getSignatureHelpItems`.

PR Close #41581

This commit implements signature help in the Language Service, on top of
TypeScript's implementation within the TCB.

A separate PR adds support for translation of signature help data from TS'
API to the LSP in the Language Service extension.

PR Close #41581