Improved syntax highlight system thanks to @liquidev and @adamharrison.
Using the new system we provide more a accurate syntax highlighting for Lua, C and C++.
Other syntax improvements contributed by @vincens2005.

On macOS declare minimum OS version as 10.13 and request permission to access Documents folder.

Move to JetBrains Mono and Fira Sans fonts for code and UI respectively.
Thet are provided under the SIL Open Font License, Version 1.1.
See doc/licenses.md for license details.

Fixed bug with fonts and rencache module.
Under very specific situations the application was crashing due to invalid memory access.

Add documentation for keymap binding, thanks to @Janis-Leuenberger.

Added a contributors page in doc/contributors.md.