doc: add a missing anchor to error codes

Only one error code was missing an appropriate anchor, and it was
ERR_SCRIPT_EXECUTION_TIMEOUT. This commit adds that missing anchor.

This is a part of the fixes hinted by #21470, which includes some tests
for error codes usage and documentation and enforces a stricter format.

PR-URL: #21483
Refs: #21470
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/errors.md

@@ -1407,6 +1407,7 @@ An attempt was made to `require()` an [ES6 module][].
 Script execution was interrupted by `SIGINT` (For example, when Ctrl+C was
 pressed).
 
+<a id="ERR_SCRIPT_EXECUTION_TIMEOUT"></a>
 ### ERR_SCRIPT_EXECUTION_TIMEOUT
 
 Script execution timed out, possibly due to bugs in the script being executed.