readtags: add tr operator

[masatake]

An example session:

[yamato@dev64]~/var/ctags-github/Tmain/readtags-formatter-op-tr.d% tail -1 output.tags
__foo	input.cpp	/^int __foo(void) { return 0; }$/;"	f	typeref:typename:int
[yamato@dev64]~/var/ctags-github/Tmain/readtags-formatter-op-tr.d% ../../readtags -t output.tags -F '(list (tr $name "_$") #t)' -l
$$foo

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.84%. Comparing base (bd0235c) to head (de25509).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
dsl/dsl.c	92.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4106   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files         239      239           
  Lines       58603    58628   +25     
=======================================
+ Hits        50306    50329   +23     
- Misses       8297     8299    +2     

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