Exclude 'target' and '.DS_Store' from checkstyle coverage

[alexjpwalker]

What is the goal of this PR?

We now exclude target and .DS_Store from checkstyle coverage tests.

What are the changes implemented in this PR?

target is typically a generated folder (e.g. by cargo). .DS_Store is generated by Finder on a Mac. They should be excluded from checkstyle coverage.

NB: Fixing #253 would remove the need for this PR, but it's harder to do.

[typedb-bot]

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