fix(parse): map Token.EXTERN in the Parse.parse_file token bridge

parse.ml's Token->Parser match (next_token) was missing the EXTERN
case. lib/dune demotes partial-match (warning 8) from error, so the
non-exhaustive match compiled but raised Match_failure at RUNTIME the
moment any `extern` token reached Parse.parse_file — i.e. on every
FFI-heavy file (all of stdlib/Deno, Vscode, Network, Sqlite, Crypto,
... use `extern fn`/`extern type`). Parser.EXTERN already existed;
this just wires the one missing arm.

Effect: Parse.parse_file (the public string/file parse API) now
handles extern-bearing sources instead of crashing. Verified the
previously-crashing stdlib/Deno|Vscode|Network now parse OK; full
dune gate green (257/257), zero regression.

Surfaced by the affinescript#218 record-migration codemod, which
relies on Parse.parse_file and was blocked on stdlib by this.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lib/parse.ml

@@ -85,6 +85,7 @@ let next_token state () =
     | Token.PUB -> Parser.PUB
     | Token.AS -> Parser.AS
     | Token.UNSAFE -> Parser.UNSAFE
+    | Token.EXTERN -> Parser.EXTERN
     | Token.ASSUME -> Parser.ASSUME
     | Token.SELF_KW -> Parser.SELF_KW
     | Token.TRANSMUTE -> Parser.TRANSMUTE