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/lower/testdata/array/function_param.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/array/function_param.carbon
 
-fn F(arr: [i32; 3], i: i32) -> i32 {
+fn F(arr: array(i32, 3), i: i32) -> i32 {
   return arr[i];
 }