Modular JAR: Add module-info module descriptor and require Java 9+

[robinst]

We've waited long enough for the ecosystem and tools such as Android to catch up. Making it proper modular means people can run jlink on it. Fixes #125.

[ice1000]

Nice!! Oh yeah!!

[ice1000]

Thank you very much!!! You're the real hero!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.96970% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 94.78%. Comparing base (870be81) to head (af61f14).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #286      +/-   ##
============================================
- Coverage     94.81%   94.78%   -0.04%     
  Complexity      252      252              
============================================
  Files           130      131       +1     
  Lines          4124     4119       -5     
  Branches        606      604       -2     
============================================
- Hits           3910     3904       -6     
- Misses          106      107       +1     
  Partials        108      108              

Files	Coverage Δ
...mark/ext/gfm/tables/internal/TableBlockParser.java	98.69% <100.00%> (ø)
...gfm/tables/internal/TableMarkdownNodeRenderer.java	97.82% <100.00%> (ø)
...nt/matter/internal/YamlFrontMatterBlockParser.java	94.91% <100.00%> (ø)
...java/org/commonmark/internal/BlockQuoteParser.java	100.00% <100.00%> (ø)
...src/main/java/org/commonmark/internal/Bracket.java	100.00% <ø> (ø)
...org/commonmark/internal/FencedCodeBlockParser.java	98.33% <100.00%> (ø)
...in/java/org/commonmark/internal/HeadingParser.java	98.63% <100.00%> (ø)
.../java/org/commonmark/internal/HtmlBlockParser.java	100.00% <100.00%> (ø)
...g/commonmark/internal/IndentedCodeBlockParser.java	92.59% <100.00%> (ø)
...java/org/commonmark/internal/InlineParserImpl.java	97.34% <100.00%> (ø)
... and 17 more

[robinst]

@noties could you have a look at these and other changes in the Android directory? The build is green with these changes, but I'm not sure I did everything properly.

There was one problem of using String#repeat (introduced in Java 11), which is not yet available on the minimum API level, which the Android build detected. I changed the code not to use that for now, and now the build is green.