adaptation to resilient v4.1

[Omikhleia]

Closes #2

Fixed:

• Using the frontmatter/mainmatter divisions in the master document to avoid having to tweak counters and folio styles manually via SIL commands
• Using the :_FANCYTOC_: predefined symbol instead of a SIL command
• Not loading packages.fancytoc any longer (since resilient v3, it's package.resilient.fancytoc and it's loaded if needed, i.e. when the above symbol is invoked.

Looking at the Djot files, there are only two remaining uses of SIL commands

• One for a skip
• One to disable headers in the frontmatter.

Maybe the second one (at least) could rather be a styling option in resilient. I'll keep that in mind, whenever I revisit the styling paradigm.

[Omikhleia]

Draft until resilient v4.1 is released, see Omikhleia/resilient.sile#187

[ul]

I extracted resilient version into .resilient-version, and enabled PDF build for the branches. We can now test in CI whether the build is successful, and even inspect resulting PDF in the artefacts.

[Omikhleia]

Draft until resilient v4.1 is released

Ready to go :)