Support importing types across Turbo Module specs

[christophpurrer]

Summary:

Changelog:

[General] [Added] - Add cross-file type import support for React Native Turbo Module codegen

Turbo Module JavaScript specs previously required all types (enums, type aliases)
to be defined in the same file. This forced developers to duplicate type
definitions when sharing them across multiple modules.

This diff adds an optional importedTypes parameter to parseString() that
allows the parser to resolve types defined in external files. The approach:

• parseString() accepts an optional TypeDeclarationMap of imported types,
  which are merged with local types (local definitions take precedence).
• parseFile() automatically resolves relative import type declarations by
  parsing the source files and extracting their type AST nodes.
• A shared resolveImportedTypes() utility handles filesystem resolution for
  both Flow and TypeScript parsers.
• getImportsFromAST() extracts type-only import declarations from the AST,
  filtering out value imports to avoid unnecessary filesystem I/O.
• The flow-schema OTA safety tool is updated to build imported types from its
  existing import map infrastructure and pass them to the codegen parser.

Code generators require zero changes since they consume the flat schema
(enumMap/aliasMap), and imported types produce identical schema entries to
file-local types.

Differential Revision: D95646987

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95646987.