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/language/for/truth.wren

@@ -1,32 +1,32 @@
 class Iter {
-  new(value) { _value = value }
+  this new(value) { _value = value }
   iterate(iterator) { _value }
   iteratorValue(iterator) { "value" }
 }
 
 // False and null are false.
-for (n in new Iter(false)) {
+for (n in Iter.new(false)) {
   IO.print("bad")
   break
 }
 
-for (n in new Iter(null)) {
+for (n in Iter.new(null)) {
   IO.print("bad")
   break
 }
 
 // Everything else is true.
-for (n in new Iter(true)) {
+for (n in Iter.new(true)) {
   IO.print("true") // expect: true
   break
 }
 
-for (n in new Iter(0)) {
+for (n in Iter.new(0)) {
   IO.print(0) // expect: 0
   break
 }
 
-for (n in new Iter("")) {
+for (n in Iter.new("")) {
   IO.print("string") // expect: string
   break
 }