Style modified lines from inserted-only lines

[richardgill]

Summary: add a separate modified-line highlight so changed lines on the right side do not use the same line background as pure insertions.

Suggested API change:

require("codediff").setup({
  highlights = {
    line_insert = "DiffAdd",
    line_change = "DiffChange",
    line_delete = "DiffDelete",
    char_insert = "DiffText",
    char_delete = "DiffDelete",
  },
})

Requirements:

• Modified lines on the modified side should use a different line highlight from inserted-only lines
• Pure insertions should keep the insert highlight
• Modified-side line highlighting should be configurable separately from insert highlighting
• This should work in both side-by-side and inline rendering

AI disclosure: this issue and linked PR were prepared with AI assistance.
PR: #324