`@actions/core` support tree shaking when bundled in a GitHub Action.

Describe the enhancement
At the moment, when using @actions/core npm package in your own bundled GitHub action, the bundler is unable to tree shake unused code from @actions/core.

Additional information

If we do the below, it should help most bundlers to determine what can be removed from @actions/core when bundling.

Add to packages/core/package.json

"sideEffects": false,

Arrow Functions

Along with using arrow functions instead of the traditional function expression

export function getInput() {
...
}

to

export const getInput = () => {
...
} 

Entry point

Add a packages/core/src/index.ts entry point that individuals exports.

For example

export {
  addPath,
  exportVariable,
  getBooleanInput,
  getInput,
  getMultilineInput,
  setCommandEcho,
  setOutput,
  setSecret
} from './core'

Circular dependency issue

The 'import' structure also has a circular dependency issue, where files sometimes import each other.

For example core.ts and oidc-utils.ts import each other.

Refactoring the structure of files and function usage would resolve this issue and help with tree shaking when bundling.

Start with packages/core/package.json and the package source files packages/core/src/core.ts and packages/core/src/oidc-utils.ts. Review the current exports and circular imports, then assess the proposed packages/core/src/index.ts entry point and sideEffects metadata against the package's bundling behavior. Done means unused @actions/core code can be tree-shaken while the documented exports continue to work.