Extract all supported functions.

[gitlost]

See #12 (comment)

Renames function map names to be more meaningful in WordPress_Code_Extractor and supports all in WordPress_Functions_Scanner::saveGettextFunctions() and errors if unmapped found.

Also errors if comments_number_link() found (not sure what to do here).

Adds tests.

[gitlost]

https://github.com/oscarotero/Gettext requires PHP 5.4!

[swissspidy]

Hmm. I simply copied comments_number_link() from the existing makepot.php file. Turns out, this isn't even an existing function in WordPress. At least not anymore.

It was added to the script over seven years ago in https://i18n.trac.wordpress.org/changeset/15325. I guess it's a remnant from the earlier days of WordPress.

We should remove it from this script as it serves no purpose.

[swissspidy]

Actually, I think we can remove support for _n_js() and _nx_js() as well since they don't exist in WordPress.

These functions were added to makepot.php in https://i18n.trac.wordpress.org/changeset/20807 to test patches for the #22229 ticket in WordPress core. I used the same function in another patch for #20491.

We can remove them now and re-add them as soon as they actually exist. However, I don't think these functions will ever make it to core in that form