Add support for custom file associations

[Dhravya]

Related to #103

Introduces custom file associations for dotenv files through settings.json, enhancing flexibility and user configuration options.

• Package.json Updates:

  • Adds a new configuration setting dotenv.files.associations to allow users to define custom file associations for dotenv files.
  • Updates activationEvents to include onLanguage:dotenv, ensuring the extension activates for custom file associations.

• Code Logic Enhancement:

  • Modifies lib/fileAssociations.js to read custom file associations from settings.json and merges them with default associations. This allows for dynamic association of files with the dotenv language based on user preferences.

• Documentation:

  • Updates README.md to include instructions on how to define custom file associations for dotenv files in settings.json, providing clear guidance for users to leverage this new feature.

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I needed this feature for allowing files like .dev.vars to be cloaked during stream. Used copilot workspace to make a PR

For more details, open the Copilot Workspace session.

[remcohaszing]

Please have a look at #105 first. It solves a deeper issue of which #103 is a symptom.