Highlight class names in new expression tree syntax

[Wilfred]

Hack is exploring expression trees, and it'd be great to have highlighting for this new syntax ready to go.

Given the code

$x = Foo`1 + 2`;

highlight Foo as a type.

Before:

After:

[Wilfred]

I'll update this to handle namespaces. I don't think it's possible to mark a pull request as 'draft' once it's published, unfortunately.

[Wilfred]

OK, this now does the right thing with all of the following:

$x = Foo`1`;
$y = \Foo`1`;
$z = bar\Foo`1`;

Should be good to merge now :)

[Wilfred]

Improved case sensitivity in regexps, so we correctly handle Foo`true`.