Touch ups & improvements to client experience

[jgunnink]

This pull request introduces several improvements to the SDK, focusing on code quality, robustness, and documentation.

• Linter Setup (ESLint):

  • Integrated ESLint with TypeScript support to enforce consistent code style and catch potential errors early.
  • Added an eslint.config.js file with recommended rules for ESLint and TypeScript.
  • Included a new lint script in package.json for easy execution.
  • Corrected all existing linting issues across the codebase.

• Enhanced Input Validation:

  • Strengthened the getFlag function by adding validation for the userContext object. It now ensures that all provided context values are of a permitted type (string, number, or boolean).
  • Added a corresponding test case to verify that the new validation correctly throws an error for invalid userContext data.

• Improved Documentation:

  • Updated the README.md to include a new Error Handling section. This section details the custom error types (APIError, InvalidResponseError, NetworkError) and provides a clear example of how to catch and handle them.