Exclude language files from phpcs linting

[johnbillion]

I noticed when trying to run composer lint:errors on my local installation that the process would run out of memory:

..........E....
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 90366752 bytes) in vendor/squizlabs/php_codesniffer/src/Runner.php on line 547

This appears to be caused by phpcs trying to scan the *.i18n.php translation files in the wp-content/languages directory, of which I have quite a few due to testing this functionality.

These files shouldn't be scanned by phpcs so can be ignored.

Trac ticket: https://core.trac.wordpress.org/ticket/59647

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Props johnbillion.

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[johnbillion]

https://core.trac.wordpress.org/changeset/58107