[c-sharp] fix Semgrep pattern parsing for top-level expressions, declarations, and ellipsis positions

[brandonspark]

Summary

Augments lang/semgrep-grammars/src/semgrep-c-sharp/grammar.js so several
modern C# pattern shapes that previously errored at parse time are now
admitted by the grammar.

Changes

• Top-level bare _expression alternative on compilation_unit so
  patterns without a trailing ; parse: $X is null,
  $X with { ... }, $X is { $P: $V },
  from $X in $Y where $C select $E.
• Top-level bare property_declaration alternative so
  public $T $P { get; init; } parses (instead of being mis-tokenized
  as local_declaration_statement + stray block).
• expression_statement extended to accept C# 7/8/9+ expression
  shapes that the upstream _expression_statement_expression rejects:
  is_pattern_expression, interpolated_string_expression,
  lambda_expression, conditional_access_expression,
  type_of_expression, element_access_expression, and
  range_expression.
• switch_expression_arm accepts $.ellipsis so
  $X switch { $P => $E, ... } parses.
• type_parameter accepts $.ellipsis so class $C<...> { } parses.

The new top-level alternatives are gated with strongly negative
prec.dynamic so ordinary C# code still resolves through the existing
previous path. Targeted conflicts entries disambiguate the small
number of unavoidable overlaps.

Tickets

• Closes LANG-478 (top-level expression patterns).
• Closes LANG-484 (top-level LINQ + property declarations).
• Partially addresses LANG-494 (augmentation parts only):
  switch-arm ... and <...> type-parameter ellipsis are fixed here.
  The I$I metavariable+identifier concatenation case from LANG-494
  is deferred — it is labeled fix:scanner-or-entrypoint upstream and
  requires scanner-level work (token.immediate for zero-width
  adjacency, or pattern-preprocessing in semgrep-core).

Test plan

• make build && make test passes (188 corpus tests, 15 new).
• tree-sitter parse spot-checked on every pattern listed in the
  three Linear tickets.
• Wait for CI on this PR.
• Land the companion semgrep-c-sharp SHA bump in
  semgrep/semgrep-c-sharp.