feat(*): add version command, build.rs script + deps

[vdice]

• Adds a -V/--version flag/command

Besides being generically useful, some tools like the spin/setup GH action automatically invoke a plugin's 'version' flag/command to check installation success. Once this plugin gains support for this flag/command, it can be installed at time of Spin setup, eg:

      - name: Install Spin
        uses: spinframework/actions/spin/setup@v1
        with:
          plugins: pluginify

(Whereas today this setup configuration would fail.)

I realize I jumped straight into implementation, only because this was fairly quick/straightforward to add. I do see that introducing the 'vergen_git2' build dep introduces a handful of others, which may not be ideal. Glad to close this (or update) to simplify!

[vdice]

(Hitting build errors related to action deprecations; separate PR to fix in #21)

[itowlson]

Good catch, thanks!