Updated PHP list of keywrods with PHP 7.4 'fn' keyword

[danon]

In PHP 7.4 there comes a new keyword, 'fn'.

I added it to the theme, I checked, and it works correctly:

• As a lambda (checked)
• As a function declaration (checked)
• As a function call (checked)
• As a classname (checked)
• As a namespace (checked)

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of +3 B (+0.2%).

file	master	pull	size diff	% diff
components/prism-php.min.js	1.94 KB	1.95 KB	+3 B	+0.2%

Generated by 🚫 dangerJS against 81e7d72

[RunDevelopment]

Please rebuild Prism by running npm run build.

Apart from that, this PR is gtg.

[RunDevelopment]

Thank you for contributing @danon!