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.

test/limit/many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3

test/limit/many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,7 +137,7 @@ class Foo {
 }
 
 class Bar is Foo {
-  this new() {
+  construct new() {
     super()
     _field129 = 129
     _field130 = 130

test/limit/too_many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3

test/limit/too_many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,7 +137,7 @@ class Foo {
 }
 
 class Bar is Foo { // expect runtime error: Class 'Bar' may not have more than 255 fields, including inherited ones.
-  this new() {
+  construct new() {
     super()
     _field129 = 129
     _field130 = 130