stage2: rework astgen command into `zig ast-check`

This addresses the use case of quickly reporting AstGen compile errors
for a file, for use with an IDE for example.

 * Rename from `zig asgen` to `zig ast-check`
 * It is now a command always available; not only in debug builds.
 * Give it usage text and proper CLI parsing.
 * Support reading from stdin when no positional arg is provided.
 * `-t` flag makes it print textual ZIR. Without this flag, it only
   provides compile errors.
 * Support `--color` parameter to override the tty detection

closes #8871

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