feat(lib/utils.js): support firefox esr

[servusdei2018]

Fixes #112

This PR adds support for detecting Firefox ESR binaries across macOS, Windows, and Linux, and cleans up utils.js for improved readability and maintainability.

User Impact

• Users can now specify -b esr or esr as the binary option, and the runner will resolve the appropriate Firefox ESR binary for their operating system.
• This enhancement ensures parity with other supported Firefox channels (e.g., Beta, Nightly, Developer Edition).

#### Code Modernization

- Replaced legacy function expressions with arrow functions where appropriate.
- Prefixed Node.js built-in modules with node: per current Node.js convention.
- Removed redundant "use strict" directives, as they are unnecessary in current Node.js environments.
- Refactored template strings in place of string concatenation for improved clarity.

[Rob--W]

Thanks for your PR!

Could you split the PR in two separate parts, one that refactors and another that introduces new features? That would make it easier to see what actually changes in support of the feature.

And although it probably does not matter in this small library, in general splitting the commit also makes it easier to do regression testing if needed, with git bisect.

Please include a unit test in https://github.com/mozilla/node-fx-runner/blob/master/test/run/test.utils.js

[servusdei2018]

cc @Rob--W tests are passing now 😄

[Rob--W]

I'm trying to verify whether the paths for ESR are correct. Could you state the Firefox versions and operating systems where you verified the accuracy of the paths included in this PR?

P.S. I had forgotten about this PR until I went through my unread notifications. If you don't get a reply within a week or two, feel free to ping me as a reminder.