DM-31253: Allow leading underscores in dataset type names

[andy-slac]

Simple update for regular expression to allow leading underscore in dataset type name.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #637 (6b71736) into main (1a2eac5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #637   +/-   ##
=======================================
  Coverage   84.13%   84.13%           
=======================================
  Files         237      237           
  Lines       30216    30216           
  Branches     5022     5022           
=======================================
  Hits        25423    25423           
  Misses       3648     3648           
  Partials     1145     1145           

Impacted Files	Coverage Δ
python/lsst/daf/butler/core/datasets/type.py	83.41% <100.00%> (ø)
tests/test_datasets.py	99.23% <100.00%> (ø)

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