feat: ignore whitespace option (diffopt+=iwhite support)

[esmuellert]

Summary

Wire the existing ignore_trim_whitespace option from the VSCode diff C library through to the user-facing config, enabling users to suppress whitespace-only changes in diffs.

Changes

• config.lua: Add diff.ignore_trim_whitespace option (default: false)
• render.lua: Pass option in both compute_and_render and compute_and_render_conflict
• auto_refresh.lua: Pass option in both regular and result-buffer diff paths
• state.lua: Pass option (previously called compute_diff with no options at all)
• README.md: Document the new option
• tests/ignore_whitespace_spec.lua: 6 test cases validating FFI behavior

Usage

require('codediff').setup({
  diff = {
    ignore_trim_whitespace = true,
  },
})

Benefits

• Declutters diffs where indentation style changed but content is the same
• Equivalent to Vim's diffopt+=iwhite
• Zero overhead — the C library already supported it, this just wires it through

Closes #232