Add option to generate blended colours

[mvllow]

Our highlight<Low,Med,High> may eventually be deprecated in favour of muted/<10,20,30,...>. It would be useful to optionally blend our colours to emulate transparency for applications that don't support our default alpha behaviour, similar to what we do in Neovim.

For example:

rose-pine-bloom --blend

# input
{ "love": "$love/50" }

# love blended against base
{ "love": "#82435b" }

I'm not sold on the naming.

[juliamertz]

Should we keep it backwards compatible? there are many themes in the org still using these colors.
Alternatively we could make a codemod to move these over

[mvllow]

I'm going to have to convert many of our repos to use bloom in the future and I don't mind handling converting highlights to muted/<number> on a case-by-case basis. Perhaps for our theme → template conversion via bloom we could change highlightLow to muted/10 etc.

(I appreciate the challenge, it's always welcome and I'm happy to be wrong/change my mind.)