[mathiasrw]

Closes #825

Stacked on #831.

Change

• A named group is now a variable with rr_ in front: rr_year for (?<year>…), and rr_year_ has a space at the end. Before it was _year, which could clash with what RexReplace uses itself: a group named _require_ broke r().
• With --js, a named group can not end with _, as rr_a_ would be both the group a_ and the group a with a space. It is a panic:

> echo -n 1x | rexreplace '(?<a>\d)(?<a_>\w)' 'rr_a' --js
 ❌ A named group can not end with _: a_
    No further processing.

Without --js there are no variables, so such a name is fine there.

The help and the README say rr_year. The tests of #799 and #815 use rr_.

Tests

test/fix/825.test.js is new. yarn test-ci passes.

After the review of 24 October

• With --output-match the code is not run, so a group name that ends with _ is fine there, and no longer a panic.
• The test of the prefix now uses a group named _require, which really clashed: with _ in front, its variable with a space was __require_, which r() uses. It fails without the fix.
• Still a panic, as decided in Use the prefix rr_ for named groups in --js, and panic on a name that ends with _ #825: a group name that ends with _ with --js, also when the code does not use the group.
• Text that only looks like a group, like \(?<a_> or [(?<a_>], is fixed in Find named groups with letters that are not ASCII in --js to close #816 #833, which reads the names from the compiled regex.

Follow-up

• The help says it shorter: "Access a named group via the name prepended with rr_. (?<year>...) gives you rr_year. A named group name can not end with _." The README line is in Update README regarding --quiet, --debug, --simulate, --sort etc. to close #821 #838.
• test/fix/825.test.js now covers names in uppercase, with a number and with $, two names that only differ by case, the cousin with a space, a name next to its $1, that _year is gone, and the panic for a_, Year_, _ and y2_ with a file that is not changed. 13 of its 15 tests fail without the fix.