fix(parser): accept resolver-modifier paths in pattern position - #10039

Summary

Adds support for $defsite variant patterns in user-defined macros. The parser now recognizes TerminalDollar as a valid start token when parsing patterns, allowing $defsite::... paths to be used in match arms within macro bodies.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

Previously, using $defsite::SomeType as a pattern in a match expression inside a user-defined macro body would fail to parse, because TerminalDollar was not recognized as a valid pattern start token. This prevented macros from referencing definition-site paths in match arms.

What was the behavior or documentation before?

A macro like:

macro unwrap_or_zero {
    ($opt: expr) => {
        match $opt {
            $defsite::Option::Some(x) => x,
            $defsite::Option::None => 0,
        }
    };
}

would fail to parse because $defsite::... was not a valid pattern.

What is the behavior or documentation after?

$defsite::... paths are now valid in pattern position within macro bodies, enabling macros to match against definition-site enum variants without diagnostics.

Related issue or discussion (if any)

Additional context

The fix is a one-line change in the pattern parser to also accept TerminalDollar as a valid start of a path pattern, alongside TerminalIdentifier. A semantic test covering the $defsite variant pattern case is included.