Add commands for parsing many files.

[doriath]

Two new commands are added to the toolkit - parse-all and summary.

This allows one to easily run the parser on many nu scripts and analyze how many of the scripts fail to parse.

Example report from running on https://github.com/nushell/nu_scripts:

$ summary (parse-all ../nu_scripts/)
Total files: 1493
Errors: 1375
Crashes: 115
Successes: 3
Sample error file: /home/doriath/code/nu_scripts/modules/formats/to-number-format.nu
Sample crash file: /home/doriath/code/nu_scripts/modules/jc/mod.nu
Slowest file: 297ms 768µs 822ns /home/doriath/code/nu_scripts/custom-completions/auto-generate/completions/mvn.nu

[kubouch]

Thanks, this looks useful. Can you add small snippets how to use these commands to their comments?

[doriath]

As recommended, added some comments. I did few more changes:

• fix the script to accept the paths to direct nu shell files, instead of just directories
• fix the formatting (use 4 space indent, to match the formatting in span command
• add export to the commands (I think it is needed so that commands are available after use toolkit.nu ?)

Also, I was wondering if it would be better for the summary to accept a pipe instead of argument? e.g. parse-all ../nu_scripts | sumary instead of current summary (parse-all ../nu_scripts)

[kubouch]

Nice, thanks! Yeah, the pipe might be nicer, but not a big deal. I'll merge as-is, but feel free to continue tweaking.