Add initial support for parsing struct patterns (#7446)

Implements parsing of struct patterns as per
https://github.com/carbon-language/carbon-lang/issues/6680.

This handles the full and short syntax given in the [design
doc](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/pattern_matching.md#struct-patterns),
but the handling of the shorthand syntax may need to change as I move on
to implementing Check support (currently the shorthand is represented as
one of `LetBindingPattern`, `VarBindingPattern`, or `VariablePattern`).

This implementation assumes that the answer to
https://github.com/carbon-language/carbon-lang/issues/7404 is that
trailing commas are allowed in the struct pattern, except for the case
where an `_` is present, in which case the next token must be the
closing brace `}`.

toolchain/parse/handle_let.cpp

@@ -19,6 +19,7 @@ auto HandleLet(Context& context) -> void {
 
   context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/false,
                               /*in_unused_pattern=*/false,
+                              /*in_field_shorthand_pattern=*/false,
                               BindingContext::ExplicitParam,
                               PrecedenceGroup::ForTopLevelPattern());
 }