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/core/function/equality.wren

@@ -1,16 +1,16 @@
 // Not structurally equal.
-IO.print(new Fn { 123 } == new Fn { 123 })  // expect: false
-IO.print(new Fn { 123 } != new Fn { 123 })  // expect: true
+IO.print(Fn.new { 123 } == Fn.new { 123 })  // expect: false
+IO.print(Fn.new { 123 } != Fn.new { 123 })  // expect: true
 
 // Not equal to other types.
-IO.print(new Fn { 123 } == 1)         // expect: false
-IO.print(new Fn { 123 } == false)     // expect: false
-IO.print(new Fn { 123 } == "fn 123")  // expect: false
-IO.print(new Fn { 123 } != 1)         // expect: true
-IO.print(new Fn { 123 } != false)     // expect: true
-IO.print(new Fn { 123 } != "fn 123")  // expect: true
+IO.print(Fn.new { 123 } == 1)         // expect: false
+IO.print(Fn.new { 123 } == false)     // expect: false
+IO.print(Fn.new { 123 } == "fn 123")  // expect: false
+IO.print(Fn.new { 123 } != 1)         // expect: true
+IO.print(Fn.new { 123 } != false)     // expect: true
+IO.print(Fn.new { 123 } != "fn 123")  // expect: true
 
 // Equal by identity.
-var f = new Fn { 123 }
+var f = Fn.new { 123 }
 IO.print(f == f) // expect: true
 IO.print(f != f) // expect: false