[typescript] fix Semgrep pattern parsing for TS-specific positions (decorators, types, interfaces, enums)

[brandonspark]

Summary

Extends lang/semgrep-grammars/src/semgrep-typescript/common/semgrep-ext.js so Semgrep patterns parse cleanly in positions that are unique to TypeScript:

• LANG-477 — standalone decorator patterns (@$DEC, @$DEC(), @$DEC(...)).
• LANG-489 — bare types and type predicates as patterns; ... and metavariables in any _type-typed slot (type alias RHS, type annotation, conditional-type sides, mapped-type value, tuple types, generic type arguments).
• LANG-500 — ... as a member of object_type (which propagates to interface_body via alias($.object_type, $.interface_body)).
• LANG-502 — ... as a member of enum_body.
• LANG-504 — <...> and <$T, ...> in type_parameters; the mapped-type RHS case is subsumed by the LANG-489 primary_type extension.

semgrep_pattern also now admits $.statement so declaration-position patterns like interface I { ... }, enum E { ... }, type T = ..., and let X: ... parse as Semgrep patterns under the __SEMGREP_EXPRESSION entry point.

Companion regeneration PRs:

• [typescript] regenerate parser for TS-specific Semgrep pattern positions semgrep-typescript#3
• [tsx] regenerate parser for TS-specific Semgrep pattern positions semgrep-tsx#2

Test plan

• make build from lang/semgrep-grammars/src/semgrep-typescript/ regenerates both typescript/ and tsx/ parsers without unresolved conflicts.
• tree-sitter parse on every reproducer from the LANG-477/489/500/502/504 tickets produces an ERROR-free tree (verified for both __SEMGREP_EXPRESSION-wrapped and bare forms).
• Existing corpus/semgrep-ext.txt cases (expression ellipsis, class-body ellipsis, function-parameter ellipsis, decorator-prefixed method patterns) still parse to the same trees.

Tickets

• LANG-477 — Standalone decorator patterns rejected
• LANG-489 — Type-position patterns rejected
• LANG-500 — Interface bodies / object type literals reject ...
• LANG-502 — Enum bodies reject ...
• LANG-504 — Mapped-type value / generic type-parameter lists reject ...

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