Add SMAPE metric

[remyogasawara]

Resolves #4218

[codecov]

Codecov Report

Merging #4220 (1c529cf) into main (c30c86b) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #4220     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        349     349             
  Lines      38256   38294     +38     
=======================================
+ Hits       38136   38174     +38     
  Misses       120     120             

Impacted Files	Coverage Δ
evalml/objectives/__init__.py	100.0% <ø> (ø)
evalml/objectives/standard_metrics.py	100.0% <100.0%> (ø)
...ata_checks_tests/test_invalid_target_data_check.py	100.0% <100.0%> (ø)
evalml/tests/objective_tests/test_objectives.py	100.0% <100.0%> (ø)
...lml/tests/objective_tests/test_standard_metrics.py	100.0% <100.0%> (ø)

[eccabay]

Looks awesome! Just a few metric-specific details that should be ironed out before merge.

[eccabay]

Awesome job!