feat: add the watch mode

[mrazauskas]

Reference #16

This PR is adding the watch mode. Just a first draft, so implementation is not yet polished. Currently it is able to detect changes in test files and to rerun them. Newly added or deleted test files are handled.

Watching dependencies and config file will be implemented later. Also I have to add testing infrastructure.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
❌ -3.28% (target: -1.00%)	❌ 29.86% (target: 95.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d7a8efd)	5453	5298	97.16%
Head commit (43c27b8)	5721 (+268)	5371 (+73)	93.88% (-3.28%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#170)	278	83	29.86%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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