[beta] More types and icons!

Additions

• You can now add/remove extension files through GMEdit!
  Technically a pretty minor feature, but spares of having to switch to IDE window just to add/remove a GML file.
• A handful of menu items now have icons!
  I am once again using famfamfam’s Silk icon set because finding icon sets that are readable, look good on different backgrounds, and have enough icons at once is a challenge.
• Added support for CustomKeyArray<K, V> (short alias: ckarray)!
  This is a formal way to handle cases where the you are using technically-not-numeric indexes - e.g. a global array of per-object metadata might be @is {ckarray<object, something>}.
• Added support for tuples!
  These are fixed-length arrays with per-index types, so [1, "2"] would be tuple<int, string>.

Improvements

• “external: <name>” in extension info now spots a “^” for clarity.
• condition ? val : undefined now returns a null<type of val>.
• GMEdit will now show basic argument help and perform argument checks for function types (function<T1, T2, RetType>).

Fixes

• Fixed a few more edge cases where an int64 could get written incorrectly.
• Fixed smart completion results getting unfairly sorted before there is anything to sort on (e.g. upon typing EnumName.)
• Fixed either<A, B> not casting to C when both A and B can cast to C.
• Fixed @self not working for GMS2.2.5 scripts.
• Fixed Combined View not adding contents to freshly created scripts (had to save again).