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.")
       }
     }

doc/site/syntax.markdown

@@ -28,8 +28,8 @@ Some people like to see all of the reserved words in a programming language in
 one lump. If you're one of those folks, here you go:
 
     :::dart
-    break class else false for foreign if import in
-    is null return static super this true var while
+    break class construct else false for foreign if import
+    in is null return static super this true var while
 
 ## Identifiers