Add oxocarbon dark and light themes

[artisanbk]

What does this PR do, and why?

Adds two new bundled themes — oxocarbon_dark and oxocarbon_light —
derived from the oxocarbon
(IBM Carbon) palette, giving users another dark/light theme pair.

Their shared palette is factored into colors_oxocarbon.py, following the
structure of the existing gruvbox theme files, and both are registered in
config/themes.py.

• oxocarbon_dark renders the carbon accents on a near-black background.
• oxocarbon_light uses a white background with a slate foreground and
  deeper blue/purple accents for contrast.

Syntax highlighting uses the Material (dark) and Lovelace (light) pygments
styles as bases, with token overrides nudged towards the oxocarbon accents.

The palette is adapted from oxocarbon.nvim (MIT License), Copyright (c) 2022
Riccardo Mazzarini; the full notice is included in colors_oxocarbon.py.

Outstanding aspect(s)

• The pygments base styles (Material/Lovelace) and the light-theme
  contrast choices are somewhat subjective — happy to adjust to reviewer
  preference.

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes #
• Partially fixes issue TRACKING: Support more Themes #1133
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

oxocarbon_dark

oxocarbon_light