Refactor the app and test units

[ufukty]

Context

• Enables testing and bundling #41 prepared the project for implementing unit tests.
• The current decomposition granularity might not be fine enough for pinpoint and independent unit testing in some places.
• There are essential features that are not currently unit tested:
  • Text matching: rule + text => range[] (vscode.Range?)
  • Config accesses #39
  • Investigate improper filtering of language config #9
  • TBA?

Problem

Lack of unit tests cause repetitive maintenance chore and might occasionally cause shipping breaking change to users.

Suggestion

Organize code for finer scoped units and implement unit tests.