Allow using regular expressions when defining license header templates

[serso]

Problem: different projects might impose different rules on license headers.
It's not uncommon, for example, that a year in the license header is the year when the source
file was created. This leads to a situation when different source files have different license
headers (at least some parts of them differ) making it impossible to use a single static
license header template to match against.

This change makes it possible to define license header templates as regexps.
An example of such template is:
// Copyright 20[0-9]{2} Artur Bosch & Contributors
which would match both
\ Copyright 2020 Artur Bosch & Contributors
and
\ Copyright 2021 Artur Bosch & Contributors

The way AbsentOrWrongFileLicense works now is controlled
by the new configuration option licenseTemplateIsRegex:
when licenseTemplateIsRegex is false the template is considered to be static
and the old matching rules apply.
If licenseTemplateIsRegex is set to true the new regexp based logic kicks in.

To avoid breakage of existing projects, the added configuration option
licenseTemplateIsRegex is disabled by default.

This commit implements a feature requested here #3100.

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[codecov]

Codecov Report

Merging #3486 (6a6a755) into master (c64b1c7) will increase coverage by 0.02%.
The diff coverage is 78.94%.

@@             Coverage Diff              @@
##             master    #3486      +/-   ##
============================================
+ Coverage     80.06%   80.09%   +0.02%     
- Complexity     2864     2868       +4     
============================================
  Files           458      458              
  Lines          8584     8596      +12     
  Branches       1663     1666       +3     
============================================
+ Hits           6873     6885      +12     
  Misses          794      794              
  Partials        917      917              

Impacted Files	Coverage Δ	Complexity Δ
...kt/rules/documentation/AbsentOrWrongFileLicense.kt	87.50% <50.00%> (-5.36%)	12.00 <7.00> (+4.00)	⬇️
...ules/documentation/LicenceHeaderLoaderExtension.kt	80.95% <86.66%> (+9.07%)	6.00 <0.00> (ø)

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[serso]

@BraisGabin @arturbosch can you please advice me what I should do with the test coverage? Codecov marks red the lines that I haven't touched. Or maybe I'm reading its report wrong?

[chao2zhang]

I would say let's ignore the codecov/path failure. Since the codecov/project is increased, I am doubtful if the codecov/patch not hitting 80% is mathematically correct.

[schalkms]

Thanks for contributing this rule! Great job! 😀
I have 1 remaining question. I think this ready to be merged.