Include git commit information in $&version

[jpco]

This is a relatively small change.

; ./es.old -c 'echo <=$&version'
es version 0.9.2 2-Mar-2022
; ./es.new -c 'echo <=$&version'
v0.9.2-248-gc7c3c42 2025-03-01

Version information is now generated automatically by the mkversion script which calls git describe --tags and git show --format=%ci.

Breaking it down to most atomic units of user-visible change:

1. Changes version format from $&version from 0.9.2 to v0.9.2 (this is just based on how git output is formatted)
2. Changes date format from $&version from 2-Mar-2022 to 2022-03-02 (also just based on git output formatting)
3. Removes the es version prefix from $&version
4. Splits version output from $&version into two separate terms, one for version string and one for commit date
5. For an es built from a non-tagged commit, changes the output from v0.9.2 to v0.9.2-(number of commits since tag)-g(commit id).

As a last piece, this changes the last line of initial.es to use the new $&version and remove the calls to date and pwd which upset reproducible-build systems like Nix.

The main point of this PR is to add the commit info, so that $&version produces output more specific than today's "built sometime in the last 3 years". I think the old setup made a lot more sense in the '90s style of development without version control, and I think this setup makes more sense now. The other changes are basically just aesthetic.

Fixes #147. This is semi-cribbed from the setup that rc has, so I guess this is also related to #1.