Add light shades, dark grey and black

[hugovk]

Fixes #31.

Includes some stuff from hfeeki/termcolor#4.

• Grey is really black, so add and advertise black, and remove grey from the docs
  • Let's keep grey as a deprecated value. We can keep it around indefinitely, there's no pressing need to remove it
• Add light shades of each colour
• Add dark grey
• Simplify constant definitions

New:

Text colors	Text highlights
black	on_black
dark_grey	on_dark_grey
light_grey	on_light_grey
light_red	on_light_red
light_green	on_light_green
light_yellow	on_light_yellow
light_blue	on_light_blue
light_magenta	on_light_magenta
light_cyan	on_light_cyan

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (5fc9ce9) compared to base (ec269e6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #32   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          106       105    -1     
=========================================
- Hits           106       105    -1     

Flag	Coverage Δ
macos-latest	100.00% <100.00%> (ø)
ubuntu-latest	100.00% <100.00%> (ø)
windows-latest	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_termcolor.py	100.00% <ø> (ø)
src/termcolor/termcolor.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.