Add <help-category> list and hint on usage in slangc -h

[aidanfnv]

For #7088

The output of slangc --help is very long, and although it is split into subsections based on categories, it is not immediately obvious from the wall of text that the -h option can take a category as a value to only display that category. You have to look in the "General" category near the top of the help output to find that -h can take an optional value <help-category>. Furthermore, these categories are not listed in the output.

This change adds a new value category for <help-category> values as the last help category, listing all of the preceding categories (and itself), with this message at the end of the text output advising users that they can narrow down the output of slangc -h.

Getting Help for Specific Categories
=====================================

To get help for a specific category of options or values, use: slangc -h <help-category>
See the <help-category> section above for the list of categories.

[aidanfnv]

/format

[slangbot]

🌈 Formatted, please merge the changes from this PR