Update to the BASIC Programming chapter that fills all the "holes".

[geneb]

All of the BASIC keywords that only had a summary in the table of BASIC statements & functions have been documented with this PR. Most of the content was taken from Pickled Light's version of the Commodore 64 Programmer's Reference Guide. All the content was typed in by hand, so any errors are mine.

Each keyword description that contained references to other keywords have had links to those keywords added. Typically, only the first mention of a keyword is linked, the remainder are highlighted as normal.

The Table of BASIC statements and functions has been revised to normalize the "Type" column. Prior to this change, the vast majority of BASIC statements were labeled as "Command". Many of these have been changed to "Statement" in following with how they were described in the C-64 PRG.

If references to specific keyboard keys were used, they have been enclosed in Markdown "highlight" tags (<mark></mark>). Custom CSS was added to .gh/markdown.css that does two things. It gives the highlighted items rounded corners and provides proper contrast that is "dark mode-aware". A second highlight tag, "marksq" was added that is effectively identical to the "mark
" style, but presents square corners. This was done as a way to highlight a special case that needed a letter on a reverse background, and a single custom image wouldn't really work out well. (This was in the PRINT keyword documentation.)

The downside of the custom CSS is that the github Markdown parser will not use it - I'm told for security reasons, and this is fine. However, anyone viewing the revised document may have an...odd experience when viewing the highlighted text.

The new CSS has been tested with the build_pdf.yml script that runs md-to-pdf and it does generate the correct output

The md-to-pdf output for Chapter 4 needs to be reviewed for opportunities to fix sub-optimal page breaks that were introduced due to the expanded content.