yeast: Add support for associating schemas with tree!/trees! calls. - #23120

Add hidden `tree!` and `trees!` variants that inject a schema dependency
and preserve access to the caller's `ctx` binding.

Essentially, these inner variants (`tree_with_schema!` etc.) will be
used internally with an interpolated reference to the output schema.
Thus, when we write `tree!(...)` this turns into
`tree_with_schema(SCHEMA, ...)`, which enables that local macro call to
(eventually) check the tree template against that schema.

Adds a new macro, `tree_schema!` which takes two arguments: an
identifier and the name of an output schema. When the macro is executed,
it then creates a local module with the same name as the specified
identifier, which exports `tree` and `trees` macros and also the schema.

These can then be imported in order to get access to these macros.

Declare the shared AST schema once and import its `tree!` wrapper for
Swift lowering helpers and rule bodies.

I think in general we'll be happier if all uses of `tree!`/`trees!` have
an (implicitly) associated schema. Thus, we don't need the generic
schema-less versions anymore.

Provide `tree_with_context!` and `trees_with_context!` for
macro-generated templates whose forwarded tokens cannot reliably
identify the local `ctx` binding.

This is essentially only needed if you're doing really peculiar stuff
with macros, but Copilot insisted, and it seemed harmless to add this.