Add parsetree support for wildcards in simple expressions

[dra27]

This PR first extends the parser to recognise syntax errors specifically related to invalid uses of _ in more places. It then converts these illegal _ to a new parsetree node, moving the error handling for encountering them to the type checker. The value of this change is that ppxes can both transform parsetrees which include the new node, and also use the OCaml parser to parse attribute and extension point payloads which include them.

The name Pexp_hole comes from the (highly experimental) feature in oxcaml/oxcaml#3310 which introduced it. Although the feature in OxCaml remains incomplete, we have found the node to be very useful in ppxes, and the parsing was extended in oxcaml/oxcaml#4956 entirely with ppx use in mind. We think the node is a useful addition to the parsetree, even without a language feature that directly requires it.

The PR itself is three fairly straightforward commits, which are worth reviewing separately:

• The first adds a parse-errors test which shows the existing fun_expr uses of _ displaying the existing "unexpected wildcard" parsing error, and all the additional simple_expr points where a standard "syntax error" is displayed
• The second commit (essentially taking the grammar changes of Parse Pexp_hole as a simple expression oxcaml/oxcaml#4956, without the parsetree node), extends the handling of _ to simple_expr, which can be seen in the parse-errors test output
• The final commit then creates the parse tree node, removing the hack from the parser and instead converting the error to come directly from the type checker

There are various subtleties (visible in the reference files) to do with _ and ~/? corner cases. For information, the original work predates LLM-use - Claude's responsibility in this PR was refactoring the work to be a syntax change, then a parsetree change, and adding the tests to demonstrate the effects.