add tests for list of coverage exclude patterns and maybe fix documentation (Scala 3)

[visma-alexander-maslov]

The -coverage-exclude-files and -coverage-exclude-packages are MultiStringSettings
https://github.com/scala/scala3/blob/2d0e37353defcec46206e9f0845c738286aabce5/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala#L122-L123

The , is used as a separator, at least according to
https://github.com/scala/scala3/blob/2d0e37353defcec46206e9f0845c738286aabce5/compiler/src/dotty/tools/dotc/config/Settings.scala#L239

README example uses ;

sbt-scoverage/README.md

Lines 77 to 89 in 76a39fb

	```scala
	coverageExcludedPackages := "<empty>;Reverse.*;.*AuthService.*;models\\.data\\..*"
	```
	The regular expressions are matched against the fully qualified class name, and
	must match the entire string to take effect. Any matched classes will not be
	instrumented or included in the coverage report.
	You can also exclude files and file paths.
	```scala
	coverageExcludedFiles := ".*\\/two\\/GoodCoverage;.*\\/three\\/.*"
	```

Using ; with the 2.1.0 plugin and 3.4.2 Scala does not exclude specified values. Changing it to , seems to work.
I have only tried it for coverageExcludedFiles, but I guess it works the same for both options.

Tests seems to lack this case.

[ckipp01]

PRs are welcome.

[jozic]

seems I've missed this, but the fix should be as easy as replace ; with , for Scala 3.
I can send a PR once I have some time or anyone else can contribute as well :)