Stdlib: show_source() — alias of highlight_file() (VM + AOT) (#3447)

[PurHur]

Summary

• Add show_source() as a thin delegate to highlight_file() per php-src ext/standard/url.c.
• Register in ext/standard/Module.php and AOT builtin_function_names.inc.
• Extend highlight_string.phpt compliance probe for show_source() echo/return paths.

php-src reference: ext/standard/url.c — PHP_FUNCTION(show_source) → highlight_file(INTERNAL_FUNCTION_PARAM_PASSTHRU)

Closes #3447

Verification

./script/docker-exec.sh --sync-back docs/capabilities.md -- bash -lc '
  source script/php-env.sh
  php script/capability-matrix.php --check
  vendor/bin/phpunit test/compliance/VMTest.php --filter highlight_string
'

./script/docker-exec.sh -- bash -lc '
  source script/php-env.sh
  php bin/compile.php -l /tmp/show_source_repro.php
'

VM test output:

OK (1 test, 1 assertion)

AOT lint: AOT lint OK

Capability matrix: show_source | yes | yes | yes | standard |

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