* std.zig.parse is moved to std.zig.Ast.parse
* the new function has an additional parameter that requires passing
Mode.zig or Mode.zon
* moved parser.zig code to Parse.zig
* added parseZon function next to parseRoot function
Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.