dsl: extend #/../ operator to be able to extract a matched group in the pattern

[masatake]

(#/..(...).../ string group) returns nth GROUP of the pattern
match. If the STRING doesn't match to the pattern, "", an empty string
is returned.

(#/..(...).../ string group fallback) does the same as
(#/..(...).../ string group) when matching. The expression returns
FALLBACK unless matching.

This extension is useful for generating C code with readtags.

An exapmle session:

  $ cat /tmp/bar.c
  enum {
    good_cat, bad_dog, bad_cat, good_dog,
  };
  $ ./ctags -o - /tmp/bar.c --extras=-p
  __anon764ae3fa0103    /tmp/bar.c      /^enum {$/;"    g       file:
  bad_cat       /tmp/bar.c      /^  good_cat, bad_dog, bad_cat, good_dog,$/;"   e       enum:__anon764ae3fa0103 file:
  bad_dog       /tmp/bar.c      /^  good_cat, bad_dog, bad_cat, good_dog,$/;"   e       enum:__anon764ae3fa0103 file:
  good_cat      /tmp/bar.c      /^  good_cat, bad_dog, bad_cat, good_dog,$/;"   e       enum:__anon764ae3fa0103 file:
  good_dog      /tmp/bar.c      /^  good_cat, bad_dog, bad_cat, good_dog,$/;"   e       enum:__anon764ae3fa0103 file:
  $ ./ctags -o - /tmp/bar.c | ./readtags -t - -F '(list (#/(good_|bad_)(.*)/ $name 2 "bird, the fallback") #t)' -l
  bird, the fallback
  cat
  dog
  cat
  dog

• consider the case if a wrong group number is given
• add a test case for the above item
• write this to ctags-client-tools.rst.in

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (c134547) 85.02% compared to head (3b59095) 85.05%.

Files	Patch %	Lines
dsl/dsl.c	60.00%	12 Missing ⚠️
dsl/es.c	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3768      +/-   ##
==========================================
+ Coverage   85.02%   85.05%   +0.02%     
==========================================
  Files         228      228              
  Lines       53901    53941      +40     
==========================================
+ Hits        45832    45880      +48     
+ Misses       8069     8061       -8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.