php-mode: improve existent snippets and add a few more

[easbarba]

I've improved ranging from shorter keys, meaningful naming and update syntax to php8.

As for the new ones::

class-final, construct, enumeration, variable, phpdoc-tag, ternary, namespace, try-catch, do-while, function-anonymous, function-arrow, match.

thanks.

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to the git conventions.

[hlissner]

I've cherry picked these into 3186b70 and 869175b (only needed to correct the commit messages to conform to our conventions). Thanks for your help, in any case!