feat: add version command

[wasabeef]

What does this change?

Added the --version option.

% fluttergen --version 
FlutterGen v4.0.1

[codecov]

Codecov Report

Merging #150 (d4b4b17) into main (e2cd36c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #150   +/-   ##
=======================================
  Coverage   98.93%   98.93%           
=======================================
  Files          18       19    +1     
  Lines         563      565    +2     
=======================================
+ Hits          557      559    +2     
  Misses          6        6           

Impacted Files	Coverage Δ
packages/core/lib/flutter_generator.dart	93.93% <100.00%> (+0.18%)	⬆️
packages/core/lib/settings/config.dart	100.00% <100.00%> (ø)
packages/core/lib/utils/version.dart	100.00% <100.00%> (ø)

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