Improve base syntax colors; different SpecialKey for Vim/Neovim;

[cideM]

• Improve base syntax HL groups and LSP & TS handling
  • Remove some unnecessary overrides, since the defaults can be improved instead
  • When overriding, try to link LSP to TS or the other way around
  • Add this info as a guideline to CONTRIBUTING
• Different SpecialKey for Vim and Neovim
  • In Vim it's used for listchars, in Neovim it's not so we can't use the same HL group for both