Allow @Mixin to be used for methods as well

[knutwalker]

This one allows Mixins to be used as method parameters, in particular for subcommands that are defined as methods.

[codecov-io]

Codecov Report

Merging #525 into master will increase coverage by 0.09%.
The diff coverage is 93.93%.

@@             Coverage Diff              @@
##             master     #525      +/-   ##
============================================
+ Coverage     89.01%   89.11%   +0.09%     
- Complexity      284      285       +1     
============================================
  Files             4        4              
  Lines          3906     3932      +26     
  Branches        959      964       +5     
============================================
+ Hits           3477     3504      +27     
  Misses          214      214              
+ Partials        215      214       -1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	89.14% <93.93%> (+0.1%)	160 <0> (+1)	⬆️

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