[Do not merge] Make StatisticalMeasures a [weakdep]

[ablaom]

This is test PR, which extends #909, moving StatisticalMeasures.jl to a weak dependency, adding DefaultMeasuresExt.jl package extension.

[codecov-commenter]

Codecov Report

Merging #910 (e2d15dd) into for-a-0-point-22-release (93cd92e) will decrease coverage by 1.64%.
The diff coverage is 95.65%.

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@@                     Coverage Diff                      @@
##           for-a-0-point-22-release     #910      +/-   ##
============================================================
- Coverage                     87.59%   85.96%   -1.64%     
============================================================
  Files                            41       33       -8     
  Lines                          3660     2835     -825     
============================================================
- Hits                           3206     2437     -769     
+ Misses                          454      398      -56     

Impacted Files	Coverage Δ
src/MLJBase.jl	100.00% <ø> (ø)
ext/DefaultMeasuresExt.jl	66.66% <66.66%> (ø)
src/composition/models/stacking.jl	94.82% <88.88%> (-0.50%)	⬇️
src/resampling.jl	91.76% <98.30%> (+0.32%)	⬆️
src/default_measures.jl	100.00% <100.00%> (ø)
src/utilities.jl	84.39% <100.00%> (-0.42%)	⬇️

... and 2 files with indirect coverage changes