Add document.lang

[gucong3000]

• Add document.lang
• Support for more extensions
• Infer syntax from source code as fallback

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #28   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     12    +1     
  Lines         250    283   +33     
=====================================
+ Hits          250    283   +33

Impacted Files	Coverage Δ
parser.js	100% <ø> (ø)	⬆️
parse.js	100% <100%> (ø)	⬆️
processor.js	100% <100%> (ø)	⬆️
syntax.js	100% <100%> (ø)	⬆️
index.js	100% <100%> (ø)	⬆️
get-lang.js	100% <100%> (ø)

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