Change array syntax from [T; N] to array(T, N) (#4981)

In line with the proposal in #4682, this changes the array syntax to be
array(T, N). `array` is a builtin keyword which must be followed by
parens containing two expressions and a separating comma.

The array type expression is still fully builtin, it does not forward to
a Core.Array library type yet. It merely adds the `ArrayType`
instruction, as was done with the previous syntax.

Followup work will change the instruction to reference to Core.Array,
once the library type exists and can be used directly.

---------

Co-authored-by: zygoloid <richard@metafoo.co.uk>

toolchain/parse/handle_expr.cpp

@@ -149,7 +149,7 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(State::ParenExpr);
       break;
     }
-    case Lex::TokenKind::OpenSquareBracket: {
+    case Lex::TokenKind::Array: {
       context.PushState(state);
       context.PushState(State::ArrayExpr);
       break;