feat: allow specifying date format

This enabled two new features:
* dynamically prepending the year
* linking to this week's page

Closes #2

feat: add ?do=today handling to link to today's page

So that I can create a browser bookmark always pointing to the right
page.

refactor: move syntax class to subdirectory

This will allow us to add further classes with more syntax options.

Also includes some type hardening.

refactor: replace deprecated assertions

test: add test for syntax component

test: use standard format for general test file

This now also works on PHP 8 without config files.

Initial commit