* improve error message when build manifest file is missing
* update std.zig.Ast to support ZON
* Compilation.AllErrors.Message: make the notes field a const slice
* move build manifest parsing logic into src/Manifest.zig and add more
checks, and make the checks integrate into the standard error
reporting code so that reported errors look sexy
closes#14290
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.
These declarations are now aliases of their new APIs and marked as
deprecated via doc comments:
* std.build.Builder
* std.build
* std.Build.LibExeObjStep