feat: replace 'this' keyword with 'construct' for constructor definitions across core lib, compiler, docs, and tests

Add TOKEN_CONSTRUCT as a new reserved word in the Wren compiler lexer and parser, and update the grammar rule table so that 'construct' triggers a constructor signature instead of 'this'. Modify all built-in class definitions in core.wren, wren_core.c, benchmark files, and test fixtures to use 'construct new(...)' and 'construct named(...)' syntax. Update documentation in classes.markdown and syntax.markdown to reflect the new keyword, and remove the deprecated test files for 'new' and infix class expressions.

doc/site/classes.markdown

@@ -172,18 +172,18 @@ constructor, like so:
 
     :::dart
     class Unicorn {
-      this new(name, color) {
+      construct new(name, color) {
         IO.print("My name is " + name + " and I am " + color + ".")
       }
     }
 
-The `this` before the method name makes it a constructor. The `new` isn't
+The `construct` before the method name makes it a constructor. The `new` isn't
 special. Constructors can have any name you like, which lets you clarify how it
 creates the instance:
 
     :::dart
     class Unicorn {
-      this brown(name) {
+      construct brown(name) {
         IO.print("My name is " + name + " and I am brown.")
       }
     }