[mathiasrw]

Closes #782

Stacked on #785.

Change

Flag	Short	Exit code 0 when
--expect-files	-F	at least 1 file is found
--expect-diff	-z	at least 1 file is changed
--expect-zero-diff	-Z	no file is changed
--expect-diff-all	(long only)	all files found are changed

When a check is not met, the exit code is 1. The run is as without the flags, so add --simulate to only check.

> rexreplace 'console\.log' '' 'src/**/*.js' --expect-zero-diff --simulate
src/app.js
 ❌ Expected no diff, but 1 file would change

• A change is the same as for the names that are printed, so a match that gives the same text is not a change. A rename is a change.
• The message says would with --simulate, --output and --output-match, and was or were without.
• Piped data counts as one file.
• With no files found, --expect-diff and --expect-diff-all are not met, and --expect-zero-diff is. --expect-files makes no files fail too.
• A check that is not met is an error: it is told with ❌, also with --quiet, and it is in errors in the status report, so errors.length tells if anything was wrong. It is also in checks, so the CLI gives 1 when the failed checks are the only errors.
• An error in a file still gives 2, also when a check is not met, as something went wrong.
• --expect-zero-diff with --expect-diff or --expect-diff-all is an error in the CLI (exit code 3), and a panic in the engine.
• The README has the flags, and code 1 in the table of exit codes.

Also in this PR

• The docs and the tests that are not about --replacement-js use its shorter name --js.
• The README shows how to run code from a file with r() (Add --js-file to read the code from a file #793).

Tests

yarn test-ci passes: the demo on Node and Bun (now with --expect-*), and 969 Bun tests. There is one engine test file for each flag, and test/cli/expect.test.js.