feat(agenda)!: rewrite agenda rendering and fix filters

[kristijanhusak]

This PR contains a complete rewrite of agenda logic, in preparation for custom agenda commands.
It also includes few fixes and breaking changes:

Breaking changes:

1. Agenda category/tags/headline search filters are now case sensitive to align with how Emacs orgmode works. This means that if you have tags as uppercase (for example MYPROJECT), you will need to type it like that in the search. Previously, everything was case insensitive and converted to lowercase.

Fixes:

1. Tags are rendered using virtual text instead of adding a padding from the headline title to align it. A similar idea was proposed previously in refactor(agenda): use extmarks to set tags in right column #683. This should fix Nvim tags in todo list are not correctly aligned for links #565. This could be considered a BC for users who use a long headline because of the overlap, but most likely it will be ok.
2. Filtering the agenda using the API ignores categories #483 should be fixed
3. Few other issues with agenda filters were fixed to be more aligned with Emacs orgmode.