Start on a Legacy submodule for backwards compat

[tecosaur]

As we change more code to use the StyledStrings approach, it would be nice to try to maintain compatibility with old user customisations as much as possible.

This is currently an incomplete solution, but as a start we can add the capability to load 256-color values and respect (some of) the JULIA_*_COLOR environment variables.

[StefanKarpinski]

Is this code copied from somewhere or just emulating the old stuff with a new implementation? If it's copied code, then leaving it as identical as possible is good (for comparability); if it's new code emulating old behavior, it makes more sense to comment on style.

[tecosaur]

This is new code to emulate prior behaviour, so I'm happy to take pointers on style.

[StefanKarpinski]

Don't care for the ternary operator, huh?

[StefanKarpinski]

A couple of suggested changes, one possible typo; maybe needs more tests so that typos would be caught?

[tecosaur]

Thanks for giving this another/closer look Stefan. I appreciate you giving some of your time to this.

Just addressed all of your comments, since it seems like the general idea/design here is fine I'll write some tests for this before thinking of merging.

(Aside: I may hold the opinion that the ternary operator is a mistake, and never should have been added 🌶️ 🌶️)

[tecosaur]

Just added tests, and fixed two issues that became apparent over the course of doing so.

Since there have been no comments against this, other than code style improvements (which have been acted on), I'll be going ahead and merging this shortly.