Infix file pattern

[alexander-yevsyukov]

This PR introduces FilePattern.infix property which aims to replace the prefix property.

ProtoData started using absolute file paths for types implementing ProtoDeclaration. This is to allow navigatable full file paths when reporting compilation errors. Because of this prefix pattern makes little sense because figuring out absolute paths on developer workstations would be awkward and meaningless.

Other notable changes

• The File type got (pattern).regex option prohibiting Windows path separators.
• Similarly to File, the Directory type also got validation option for requiring Unix path separators.
• Functions converting Files and Paths got names clarifying where the output is definitely absolute.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.07%. Comparing base (ff7faf1) to head (20fc512).
Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #330   +/-   ##
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  Coverage     76.07%   76.07%           
- Complexity      558      560    +2     
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  Files           192      192           
  Lines          4158     4168   +10     
  Branches        392      392           
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+ Hits           3163     3171    +8     
- Misses          863      865    +2     
  Partials        132      132