Update environment variables and resolve JSON module in tsconfig.json

[martedesco]

This pull request introduces the ability to use mocked data for the GitHub Copilot metrics, which can be toggled through a new environment variable. The changes are primarily in the README.md, .env, and src/api/GitHubApi.ts files. The most significant changes include the addition of the VUE_APP_MOCKED_DATA environment variable, updates to the setup instructions in the README.md file to include details about the new environment variable, and modifications to the getGitHubCopilotMetricsApi function to return mocked data when the VUE_APP_MOCKED_DATA environment variable is set to true.

Mock data feature introduction:

• .env: Added VUE_APP_MOCKED_DATA environment variable to toggle the usage of mocked data.
• README.md: Updated the setup instructions to include details about the new .env file setup and how to use the VUE_APP_MOCKED_DATA variable.
• src/api/GitHubApi.ts: Modified the getGitHubCopilotMetricsApi function to use the mocked data stored in a JSON file when VUE_APP_MOCKED_DATA is set to true; otherwise, it makes an API call as before. [1] [2]