Registration: Rename default block handlers as reflecting name

[aduth]

This pull request seeks to rename default block handler functions to better reflect that they operate on a block name, not a block instance, type, or otherwise. This brings default block API handlers in line with unknown type handlers, which were similarly renamed in bc4d6cd. I originally planned to include these changes in a related set of changes, but since it touched a handful of files, I felt it better to create a smaller, separate pull request.

Testing instructions:

Verify there is no regressions in the behavior of the default block (paragraph).

[codecov]

Codecov Report

Merging #2627 into master will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #2627   +/-   ##
=======================================
  Coverage   31.42%   31.42%           
=======================================
  Files         177      177           
  Lines        5407     5407           
  Branches      946      946           
=======================================
  Hits         1699     1699           
  Misses       3135     3135           
  Partials      573      573

Impacted Files	Coverage Δ
blocks/api/registration.js	100% <ø> (ø)	⬆️
editor/modes/visual-editor/block-list.js	0% <0%> (ø)	⬆️
blocks/library/paragraph/index.js	33.33% <100%> (ø)	⬆️

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[mtias]

Makes sense.