fix: Added alias for watch-file option

[ankushduacodes]

Added alias for watch-file option and updated option description to reflect recent changes to the option

Continuation of #2125. Opening this PR in response to the discussion in extension-workshop PR #950 C3

@rpl Are there any tests to check if the alias does work as expected?

[ankushduacodes]

For some reason, I am not able to see open ci job that failed (sending back a blank screen), Will try again soon to see what the issue is.

[rpl]

@ankushduacodes thanks a lot for creating a PR for the alias, just in time for including it into the new version.

About the tests: this kind of tests can be covered with a functional test, e.g. we may be able to cover it with just a couple of small changes to tests/functional/test.cli.run.js.

About the CI job that did fail:
oddly I got a white page when I tried to follow the link for the failure, but I got to the failure logs by navigating to it from the circleci homepage.
Anyway, the failure is only related to the commit message not following the convention:

There is only one commit in this pull request, we are going to check the single commit message...
Failures during changelog linting the pull request:

⧗   input: added alias for watch-file opt and updated option description to reflect recent changes related to supporting multiple files to watch #2125
✖   subject may not be empty [subject-empty]
✖   type may not be empty [type-empty]
⚠   header must not be longer than 72 characters, current length is 139 [header-max-length]

✖   found 2 problems, 1 warnings
ⓘ   Get help: https://github.com/conventional-changelog/commitlint/#what-is-commitlint

Don't panic! If the CI job is failing here, please take a look at

 - https://github.com/mozilla/web-ext/blob/master/CONTRIBUTING.md#writing-commit-messages

Updating the commit message to follow the convention will fix that linting failure.

btw, I think in this case the conventional prefix should be fix: ... or chore: ....

[rpl]

Using a "custom build process" is the most common reason for being interested into watching a specific file (or set of files) for changes, and that didn't change with the addition of the alias.
I think that in the end we may just leave the description as is.

[ankushduacodes]

Got it. Going back...

[ankushduacodes]

@rpl I added a test that does two things at the same time:

1. Check for alias's working
2. Providing multiple files in one option (Although yargs treat --watch-file file1 --watch-file file2 and --watch-file file1 file2 the same), I added this test because I had added same kind of test for watch-ignored as well.

Please let me know if any changes are required :)

[rpl]

lgtm

Thanks @ankushduacodes