fix: make config file parsing stricter and support config includes

[matchaxnb]

Description
Improve the config parsing and documentation around it.

• now support comments at the end of lines
• trims values of their trailing whitespaces
• refactor error handling in args and config so that all errors are caught at once (alas, uses a global var in the config module, but it's fair ig)
• safer integer parsing (atoi is known to not care at all for errors)
• error status is now a bool flag carried around, so parser functions are void ...()
• update moonlight.conf with a bit of docs to be explicit about parsing format

Purpose

• ensure config supports the features that are documented as working (like includes)
• avoid bad types causing issues (like putting a float in the resolution or whatever)

[matchaxnb]

Fixes #935