Highlight super and this as keywords in JS/TS/TSX (#25135)

Closes #24951

We were highlighting both as `@variable.special` however, they are
_techinically_ keywords and other editors (VSCode/WebStorm) seem to
highlight them as keywords as well.

Release Notes:

- N/A

crates/languages/src/javascript/highlights.scm

@@ -59,8 +59,8 @@
 
 ; Literals
 
-(this) @variable.special
-(super) @variable.special
+(this) @keyword
+(super) @keyword
 
 [
   (null)