implement is_offsetlike

[jbrockmendel]

This is a subset of #18817 that just implements is_offsetlike in dtypes.common (plus a nicer docstring). This needs to be implemented before some fixes can be made in indexes.datetimelike, so is worth separating from the more difficult bits of 18817.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jreback]

can you add some examples

[codecov]

Codecov Report

Merging #18823 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18823      +/-   ##
==========================================
- Coverage   91.64%   91.62%   -0.02%     
==========================================
  Files         154      154              
  Lines       51401    51408       +7     
==========================================
- Hits        47106    47104       -2     
- Misses       4295     4304       +9

Flag	Coverage Δ
#multiple	89.49% <100%> (ø)	⬆️
#single	40.83% <70%> (-0.13%)	⬇️

Impacted Files	Coverage Δ
pandas/core/ops.py	91.69% <100%> (-0.09%)	⬇️
pandas/core/dtypes/common.py	94.54% <100%> (+0.08%)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.68% <0%> (-0.11%)	⬇️
pandas/core/groupby.py	92.07% <0%> (+0.02%)	⬆️

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[jreback]

thanks!