Added prefetchIvyDeps target and offline support

[lefou]

• Updated scalafmt config
• Added prefetchIvyDeps target and offline support
• Documented new target
• Dropped support for older mill versions
• Update os-lib version to 0.7.1

[codecov-commenter]

Codecov Report

Merging #65 (e792f61) into main (5d150d0) will increase coverage by 2.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   90.24%   92.30%   +2.06%     
==========================================
  Files           5        5              
  Lines          41       39       -2     
==========================================
- Hits           37       36       -1     
+ Misses          4        3       -1     

Impacted Files	Coverage Δ
...ll/integrationtest/MillIntegrationTestModule.scala	88.46% <ø> (+3.27%)	⬆️
...e/tobiasroeser/mill/integrationtest/TestCase.scala	100.00% <0.00%> (ø)

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