[GNU ELPA] Xr version 1.21

[ELPA update]

Version 1.21 of package Xr has just been released in GNU ELPA.
You can now find it in M-x package-list RET.

Xr describes itself as:
  Convert string regexp to rx notation

More at https://elpa.gnu.org/packages/xr.html

Recent NEWS:

Version 1.21:
- Suppress false complaint about (? (+ X))
Version 1.20:
- Fix duplication removal in character alternatives, like [aaa]
- All diagnostics are now described in the README file
- Improved anchor conflict checks
Version 1.19:
- Added filename-specific checks; new PURPOSE argument to `xr-lint'
- Warn about wrapped subsumption, like \(A*C[AB]*\)+
- Improved scope and accuracy of all subsumption checks
- Warn about anchors in conflict with other expressions, like \(A$\)B
Version 1.18:
- Fix test broken in Emacs 26
Version 1.17:
- Performance improvements
Version 1.16:
- Translate [^\n] into nonl
- Better character class subset/superset analysis
- More accurate repetition subsumption check
- Use text quoting for messages
Version 1.15:
- Warn about subsuming repetitions in sequence, like [AB]+A*
Version 1.14:
- Warn about repetition of grouped repetition
Version 1.13:
- More robust pretty-printing, especially for characters
- Generate (category CHAR) for unknown categories
Version 1.12:
- Warn about branch subsumption, like [AB]\|A
Version 1.11:
- Warn about repetition of empty-matching expressions
- Detect `-' not first or last in char alternatives or skip-sets
- Stronger ad-hoc [...] check in skip-sets
Version 1.10:
- Warn about [[:class:]] in skip-sets
- Warn about two-character ranges like [*-+] in regexps
Version 1.9:
- Don't complain about [z-a] and [^z-a] specifically
- Improved skip set checks
Version 1.8:
- Improved skip set checks
Version 1.7:
- Parse skip-sets, adding `xr-skip-set', `xr-skip-set-pp' and
  `xr-skip-set-lint'
- Ad-hoc check for misplaced `]' in regexps
Version 1.6:
- Detect duplicated branches like A\|A
Version 1.5:
- Add dialect option to `xr' and `xr-pp'
- Negative empty sets, [^z-a], now become `anything'
Version 1.4:
- Detect overlap in character alternatives
Version 1.3:
- Improved xr-lint warnings
Version 1.2:
- `xr-lint' added