fix: respect NO_COLOR and non-TTY output for --check diffs

[bmish]

Summary

• Honor NO_COLOR / non-TTY stderr (and FORCE_COLOR) when printing --check diffs from jest-diff, so CI and redirected logs no longer get unconditional ANSI.
• Add a regression test that stubs NO_COLOR=1 and asserts stderr contains no ESC sequences.
• Document the behavior on the README --check option row.

Test plan

• npm run lint
• npm test
• Manually: run --check against out-of-date docs with NO_COLOR=1 and confirm plain diffs; with FORCE_COLOR=1 confirm colors still apply

Made with Cursor

Context

From the post-#987 standards audit: the --check diffs from jest-diff are the only ANSI output this tool emits, and they were emitted unconditionally — including into piped CI logs and with NO_COLOR set. Follows the https://no-color.org convention with FORCE_COLOR override, keyed off stderr (diffs go to console.error).