feat: replace new keyword with this constructor syntax and ClassName.new() calls across core library and docs

Replace all uses of the `new` reserved word with explicit `this new()` constructor definitions and `ClassName.new()` instantiation calls in builtin/core.wren. Update all documentation files (classes.markdown, fiber.markdown, fn.markdown, sequence.markdown, error-handling.markdown, expressions.markdown, fibers.markdown, functions.markdown) to reflect the new constructor syntax, removing `new` keyword examples and replacing them with `ClassName.new()` patterns and `this new()` definitions.

test/limit/long_function.wren

@@ -1,4 +1,4 @@
-new Fn {
+Fn.new {
   IO.print(1) // expect: 1
   IO.print(2) // expect: 2
   IO.print(3) // expect: 3

test/limit/many_constants.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   1
   2
   3

test/limit/many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  new {
+  this new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -259,4 +259,4 @@ class Foo {
   }
 }
 
-var foo = new Foo // expect: 255
+var foo = Foo.new() // expect: 255

test/limit/many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  new {
+  this new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,8 +137,8 @@ class Foo {
 }
 
 class Bar is Foo {
-  new {
-    super
+  this new() {
+    super()
     _field129 = 129
     _field130 = 130
     _field131 = 131
@@ -274,7 +274,7 @@ class Bar is Foo {
   }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 bar.foo // expect: 1
 // expect: 128
 bar.bar // expect: 129

test/limit/too_many_constants.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   1
   2
   3

test/limit/too_many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  new {
+  this new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -259,4 +259,4 @@ class Foo {
   }
 }
 
-var foo = new Foo
+var foo = Foo.new()

test/limit/too_many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  new {
+  this new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,8 +137,8 @@ class Foo {
 }
 
 class Bar is Foo { // expect runtime error: Class 'Bar' may not have more than 255 fields, including inherited ones.
-  new {
-    super
+  this new() {
+    super()
     _field129 = 129
     _field130 = 130
     _field131 = 131
@@ -275,6 +275,6 @@ class Bar is Foo { // expect runtime error: Class 'Bar' may not have more than 2
   }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 bar.foo
 bar.bar