scheme: Add racket support to scheme parser

[plv]

Addressing #1877 by adding support for the .rkt extensions. Change is simple because syntax is the same. Tested locally on some Racket source.

[coveralls]

Coverage remained the same at 86.311% when pulling dbeba03 on plv:racket-support into 47de0ff on universal-ctags:master.

[codecov-io]

Codecov Report

Merging #2315 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2315   +/-   ##
=======================================
  Coverage   86.18%   86.18%           
=======================================
  Files         177      177           
  Lines       35863    35863           
=======================================
  Hits        30910    30910           
  Misses       4953     4953

Impacted Files	Coverage Δ
parsers/scheme.c	100% <ø> (ø)	⬆️

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[masatake]

Thank you!