src: fix undefined script name in error source

[legendecas]

If an error is not caught, the file name and source code line where the error was thrown is printed. However, for unnamed eval scripts, a placeholder instead of undefined should be print:

undefined:3
  throw new Error('error in unnamed script');
  ^

Error: error in unnamed script
    at eval (eval at <anonymous> (/dev/nodejs/node/test/fixtures/errors/throw_in_eval_unnamed.js:5:1), <anonymous>:3:9)

This PR prints a placeholder <anonymous_script> for such errors:

<anonymous_script>:3
  throw new Error('error in anonymous script');

[joyeecheung]

LGTM with a suggestion.

[joyeecheung]

Suggested change

	filename_string = "<unnamed_script>";
	filename_string = "<anonymous>";

Which is what Chrome DevTools use, or I think <anonymous_script> is somewhat easier to understand. It would be even clearer if it's <eval_script> but I suspect this can be hit in other non-eval cases so can be tricky.

[legendecas]

I didn't use <anonymous> because it has been used to refer to anonymous functions in the error stacks just below the sources. I think it could be confused with anonymous scripts and functions.

<anonymous_script> looks good to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.12%. Comparing base (52c6449) to head (996e2c3).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #56502      +/-   ##
==========================================
- Coverage   89.12%   89.12%   -0.01%     
==========================================
  Files         662      662              
  Lines      191555   191560       +5     
  Branches    36856    36854       -2     
==========================================
- Hits       170730   170721       -9     
- Misses      13696    13701       +5     
- Partials     7129     7138       +9     

Files with missing lines	Coverage Δ
src/node_errors.cc	64.67% <100.00%> (+1.17%)	⬆️

... and 28 files with indirect coverage changes

[jasnell]

LGTM with @joyeecheung's suggestion

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/64411/