created additional validators to differentiate between enabling standard and preview features

[MysterAitch]

fixes #2701

Note that the API or Behaviour Change label is added because some features that were previously LanguageLevel.JAVA_14 are now LanguageLevel.JAVA_14_PREVIEW (and similar for other language levels that introduced preview features).

When merging this, I'll be sure to squash it rather than keeping this messy history!

• Fixed some tests to reflect the above
• Removed some empty post-processors
• Moved some packages around for the validators
• Added some documentation is places to group together some of the validators and explain why they're there (or not)
• Some validations for record declarations and sealed classes are there as a "TODO" reminder

[codecov]

Codecov Report

Merging #3015 (83c6120) into master (7a55b9b) will increase coverage by 0.021%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master     #3015       +/-   ##
=============================================
+ Coverage   50.424%   50.446%   +0.021%     
=============================================
  Files          449       450        +1     
  Lines        25067     25096       +29     
  Branches      3790      3790               
=============================================
+ Hits         12640     12660       +20     
- Misses       11450     11458        +8     
- Partials       977       978        +1     

Flag	Coverage Δ
AlsoSlowTests	50.446% <100.000%> (+0.021%)	⬆️
javaparser-core	50.446% <100.000%> (+0.021%)	⬆️
jdk-10	50.434% <100.000%> (+0.009%)	⬆️
jdk-11	50.446% <100.000%> (+0.021%)	⬆️
jdk-12	50.446% <100.000%> (+0.021%)	⬆️
jdk-13	50.446% <100.000%> (+0.021%)	⬆️
jdk-14	50.446% <100.000%> (+0.021%)	⬆️
jdk-15	50.446% <100.000%> (+0.021%)	⬆️
jdk-8	50.444% <100.000%> (+0.021%)	⬆️
jdk-9	50.446% <100.000%> (+0.021%)	⬆️
macos-latest	50.434% <100.000%> (+0.021%)	⬆️
ubuntu-latest	50.434% <100.000%> (+0.021%)	⬆️
windows-latest	50.434% <100.000%> (+0.021%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...or/language_level_validations/Java12Validator.java	100.000% <ø> (ø)
...or/language_level_validations/Java13Validator.java	100.000% <ø> (ø)
...r/language_level_validations/Java1_1Validator.java	100.000% <ø> (ø)
...r/language_level_validations/Java1_2Validator.java	100.000% <ø> (ø)
...r/language_level_validations/Java1_3Validator.java	100.000% <ø> (ø)
...r/language_level_validations/Java1_4Validator.java	100.000% <ø> (ø)
...tor/language_level_validations/Java5Validator.java	100.000% <ø> (ø)
...tor/language_level_validations/Java6Validator.java	100.000% <ø> (ø)
...tor/language_level_validations/Java7Validator.java	100.000% <ø> (ø)
...tor/language_level_validations/Java8Validator.java	100.000% <ø> (ø)
... and 34 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a55b9b...83c6120. Read the comment docs.