This commit removes `gawk`-specific extensions in Grawkit, making it work for any POSIX-compatible
AWK, with `gawk`, `nawk`, `busybox awk`, and `goawk` passing the test suite successfully.
The largest changes are related to removing `match` extensions for capturing groups; these were used
for parsing command-lines, and have now being handled in the `parse_command` function in a more robust
way.
Other small changes have been made to the included `Makefile` for allowing tests to run against
different AWK implementations.
This version has support for adding commits on a single branch using
`git commit`, and enough scaffolding for adding additional commands and
accompanying tests easily.
