Add fennel support

[tranzystorekk]

Adds support for fennel language source code files *.fnl and fennel repl config files .fennelrc.

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TODO

• Use cargo fmt
• Add necessary tests
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (41893ae) 84.63% compared to head (c8c9ab4) 84.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #992   +/-   ##
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  Coverage   84.63%   84.63%           
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  Files          53       53           
  Lines        5078     5078           
=======================================
  Hits         4298     4298           
  Misses        780      780           

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