Add syntax highlighting for PRQL

[vanillajonathan]

Description of changes:
Adds a syntax highlighting mode for the PRQL query language.

PRQL is a modern language for transforming data — a simple, powerful, pipelined SQL replacement.
https://prql-lang.org/
https://github.com/PRQL/prql

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6c18338) 87.25% compared to head (cf0755f) 87.26%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5307   +/-   ##
=======================================
  Coverage   87.25%   87.26%           
=======================================
  Files         567      569    +2     
  Lines       45368    45394   +26     
  Branches     6930     6930           
=======================================
+ Hits        39587    39613   +26     
  Misses       5781     5781           

Flag	Coverage Δ
unittests	87.26% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
src/mode/prql.js	100.00% <100.00%> (ø)
src/mode/prql_highlight_rules.js	100.00% <100.00%> (ø)

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[akoreman]

Thanks for your contribution.

[snth]

Thanks