[morph.ai] Fixed PLAYGROUND-PR-3000 -- Fix highlighting of PHP arrow functions

[MorphCodeBot]

This PR fixes the incorrect syntax highlighting of PHP arrow functions that use the 'fn' keyword.

Problem:

• Arrow functions using 'fn' were being incorrectly treated as regular function declarations
• This caused comments in arrow functions to be highlighted as function titles

Changes:

• Added 'fn' to the PHP keywords list
• Removed 'fn' from function detection keywords

Example of fixed highlighting:

$fn1 = fn($x) => $x + $y;

The arrow function is now correctly highlighted with:

• 'fn' as a keyword
• Parameters and variables properly recognized
• No incorrect title detection

This change improves the syntax highlighting accuracy for modern PHP code that uses arrow functions.

Additional ticket processing details