migrate the `grunt test` task to package.json. `npm test` is the new
alternative command.

this is the first ( and easiest ) step to eliminating `grunt` as a
dependency for the project.

There were 2 divergences in the `manifest.json` files :
- `"applications"` block in the firefox manifest which specified the
  minimum supported version of the extension in the browser
- different icon sizes in the default `manifest.json`

This divergence seemed trivial to be maintained separately. Merging and
running the extension from the merged `manifest.json` file did not throw
any major errors in the running of the extension on Firefox as well as
Chromium.

It does not seem sensible to maintain 2 separate `manifest.json` files
for the extension unless there are other chromium-based browsers who use
the `"applications"` block specifically for something else or some other
browser that accepts only one icon in the manifest.

Now that the only minor divergence in the source code have been merged,
the `src` directory can be used as the single source of truth for both
source code as well as distribution code. There is nothing that needs to
be compiled or transpiled that would require a separate dedicated
distribution directory.

because web-ext run already watches, live reloads and also serves the web
extension to the desired browser described by `npm run -- serve:firefox`
or `npm run -- serve:chromium`

`npm` has a built-in `pack` command that can compress the repository (
or specific `files` in the repository ) and generate a tarball.
Additionally, it also adds some default files like the README, LICENSE
and CHANGELOG for posterity.
The output tarball is also aptly named in the format
`<project-name>-<version-string>.tgz`

reference : https://stackoverflow.com/a/55220535

While the tarball can be generated by running the `npm pack` command,
for sake of backward-compatibility of existing `npm` scripts, it has
been assigned to the `package` script replacing the `grunt package`
alias.

remove all remaining traces of grunt from the project

`a0bc2c0` blindly migrated the `grunt` alias `test` without looking at
the script's semantics.