This brings a few advantages:
- Use of float instead of strings to represent float values,
- Use of structure settings, and
- Better type checking for some settings
Also add thiagokokada as codeowner of picom.
Remove `stateVersion`, `username`, and `homeDirectory` as they can be
set in the configuration directly. Together with the previous commit,
this makes setting `stateVersion` explicitly mandatory.
Also replace `configuration` by `modules`, which expects a list of
Home Manager modules.
Also remove `system` which was made useless by #3028.
The user should always explicitly set the state version they wish to
use. Indeed, the configuration generated by the Home Manager install
script has set this option for a long time. This removal should
therefore not affect many users.
