fix(highlights): preserve character group attributes

[richardgill]

Summary

Preserve the full highlight definition when char_insert or char_delete references a highlight group, instead of copying only its effective background.

Motivation: I wanted to style char insert text white like github diffs. And this was preventing me from doing that.

Example

vim.api.nvim_set_hl(0, "MyCharInsert", {
  fg = "#ffffff",
  bg = "#005f00",
  bold = true,
  nocombine = true,
})

require("codediff").setup({
  highlights = {
    char_insert = "MyCharInsert",
  },
})

Given this change:

before: local status = "pending"
after:  local status = "complete"

                       ^^^^^^^^  character-level insertion

Before this fix:

CodeDiffCharInsert:
  bg=#005f00

Only the background was copied. The configured foreground, bold, and nocombine attributes were lost.

After this fix:

CodeDiffCharInsert:
  fg=#ffffff bg=#005f00 bold nocombine

Resulting rendering:

after:  local status = "complete"
                       ^^^^^^^^
                       white, bold text on #005f00 from MyCharInsert hl

What changed

• Preserve foreground, special color, GUI/cterm styles, and nocombine from configured character highlight groups.
• Materialize reversed GUI and cterm colors before removing the reverse flags.
• Use fallback colors only when the configured group has no background.
• Keep line highlights background-only so syntax highlighting remains visible.
• Keep direct colors and automatically derived character highlights background-only.
• Re-resolve character group attributes after ColorScheme changes.

Behavior change

Named character highlight groups now define the complete appearance of changed characters. Users who want a background override without replacing syntax foreground or styles can continue to use a direct color or a custom background-only highlight group. Default and automatically derived character highlights are unchanged.

Test coverage

Added coverage for preserved attributes, reverse colors, line and direct-color behavior, derived highlights, and ColorScheme updates.

Fixes #446