Add circuitpython-mpy-cross

This wraps the process of building or downloading mpy-cross for a specified
version, then running it with specified arguments.

e.g.,
```
$ circuitpython-mpy-cross --circuitpython-version 9.x -- --version
CircuitPython 9.0.0-alpha.2 on 2023-10-27; mpy-cross emitting mpy v6.1
```

circuitpython_build_tools/scripts/circuitpython_mpy_cross.py

@@ -0,0 +1,21 @@
+import subprocess
+
+import click
+
+from ..target_versions import VERSIONS
+from ..build import mpy_cross
+
+@click.command(context_settings={"ignore_unknown_options": True})
+@click.option("--circuitpython-version", type=click.Choice([version["name"] for version in VERSIONS]))
+@click.option("--quiet/--no-quiet", "quiet", type=bool, default=True)
+@click.argument("mpy-cross-args", nargs=-1, required=True)
+def main(circuitpython_version, quiet, mpy_cross_args):
+    version_info, = [v for v in VERSIONS if v["name"] == circuitpython_version]
+    mpy_cross_exe = str(mpy_cross(version_info, quiet))
+    try:
+        subprocess.check_call([mpy_cross_exe, *mpy_cross_args])
+    except subprocess.CalledProcessError as e:
+        raise SystemExit(e.returncode)
+
+if __name__ == '__main__':
+    main()

setup.py

@@ -18,5 +18,6 @@
       entry_points='''
         [console_scripts]
         circuitpython-build-bundles=circuitpython_build_tools.scripts.build_bundles:build_bundles
+        circuitpython-mpy-cross=circuitpython_build_tools.scripts.circuitpython_mpy_cross:main
       '''
       )