chore(#192): generate from css class names

[alex-courtis]

fixes #192

feeds #414

Still contains validation programs to check against master.

Icons are mostly unchanged; seti refactor etc. can come later.

Changes, for classes with no nerd-font icon:

lua scripts/gen-check.lua
default icons_by_filename
default icons_by_file_extension
sig                       'λ' ~= '󰘧'
nu                        '>' ~= ''
sml                       'λ' ~= '󰘧'
cob                       '⚙' ~= ''
cpy                       '⚙' ~= ''
cobol                     '⚙' ~= ''
cbl                       '⚙' ~= ''
mo                        '∞' ~= '󰛤'
default icons_by_operating_system
light icons_by_filename
light icons_by_file_extension
sig                       'λ' ~= '󰘧'
nu                        '>' ~= ''
sml                       'λ' ~= '󰘧'
cob                       '⚙' ~= ''
cpy                       '⚙' ~= ''
cobol                     '⚙' ~= ''
cbl                       '⚙' ~= ''
mo                        '∞' ~= '󰛤'
light icons_by_operating_system

• move all generation into generate_colors.lua
• remove luarocks bits from CI
• remove gen-check.lua before merge
• update CONTRIBUTING.md

[gegoune]

Nice work!

For autogenerated files we could consider adding https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github.