Add checkstyle plugin

[marschattha]

One thing of note is checkfile needs a config file to run so set to suggested = "config".

In the docs example the config file in most places is just config.xml which seems to generic, so I put it as checkstyle.xml for now.

[qltysh]

Diff Coverage for cli: The code coverage on the diff in this pull request is 100.0%.

Total Coverage for cli: This PR will not change total coverage.

File Coverage Changes

Path	File Coverage Δ	Indirect
qlty-check/src/tool/ruby/sys/macos.rs	2.6	√
qlty-check/src/tool/download.rs	0.7	√
qlty-check/src/executor/driver.rs	-0.9
qlty-check/src/tool/ruby/sys/linux.rs	-1.3	√

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• Diff Coverage: Coverage for added or modified lines of code (excludes deleted files). Learn more.

• Total Coverage: Coverage for the whole repository, calculated as the sum of all File Coverage. Learn more.

• File Coverage: Covered Lines divided by Covered Lines plus Missed Lines. (Excludes non-executable lines including blank lines and comments.)

  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.

[brynary]

Looking good!