feat: add testing & strict secret scanning

[kennething]

Adds unit/integration testing with Mocha, and these changes in the changelog:

Added

• Added enableStrictSecretScanning setting (and corresponding toggleStrictSecretScanning command) that, when enabled, makes secret scanning skip files that don't have a common secret indicator (like "API_SECRET").
• Added .svelte, .txt, and .toml to the default list of scanned file extensions.

Changed

• GitGerbil will now wait for up to 5 seconds to detect a git repo when activating instead of immediately failing.
• File extensions like .test.ts will be correctly detected as .ts files now and scanned if the base extension is in the list of scanned file types.
• Submitting an empty field when running gitgerbil.setScannedFileTypes will now reset to the default list of scanned file types instead of an empty list.
• Updated the README to mention gitgerbil-ignore-file.
• .env.example files will no longer be flagged by file path scanning. (regex erroneously matches .env.example #2)
• Fixed file name detection for files nested in subdirectories.

Removed

• Replaced enable and disable commands (i.e. gitgerbil.enableSecretScanning) with toggle commands.
  • gitgerbil.toggleFilePathScanning
  • gitgerbil.toggleSecretScanning
  • gitgerbil.toggleStrictSecretScanning
  • gitgerbil.toggleCommentScanning
• Removed SQL from comment scanning.