feat(developer): package compiler should convert .md files to .htm at compile time 🎺

[mcdurdin]

Relates to #9351.

If any of the metadata fields ReadmeFile, LicenseFile, WelcomeFile (see #9478) are listed in a package, and the file it refers to is a .md file, then it should be converted to .htm during the package compile, and the kmp.json file should refer to the .htm, and only the .htm should be included.

Note: don't forget to check for filename conflict when renaming -- if there is also a license.htm in the file, that will cause grief. (Case insensitive comparison reqd)

Another set of questions about multi-file documentation:

• how do we handle internal links between .md files in packages? Need to rewrite the links generated from the .md and build list of all .md files to convert from that?
• if we rename arbitrary.htm (whether generated from arbitrary.md or not) to welcome.htm for WelcomeFile, for better backward compatibility, then links to arbitrary.htm from other documentation files would also need to be updated. Suggestion 1: copy to welcome.htm rather than rename?

Question: header and footer for the .htm, as the .md will only be the content.

This relates to the kmp info.description field, which is also .md, although inline and without header and footer. This particular field also disables inline html, but the isolated .md files should allow inline html.

[mcdurdin]

Note for @keymanapp/developers: please do not attempt to consume LICENSE.md when found in a .kmp, even if we have .kmp files in the wild that have LICENSE.md files. We'll only start to present license metadata from packages once we implement this feature in the package compiler -- it is not appropriate for us to add Markdown parsers to the Keyman apps.

[mcdurdin]

Additional design thought: we should make it possible to specify a single custom CSS for the .md files in the packages, and include a standard one in Keyman Developer. (If we tag the body of the generated files with readme, welcome, and license class names, the single .css could cover all three files easily enough.)