This pull request seeks to resolve inconsistencies between block registration validation and name parsing expectations. Specifically:

• Registration requires that a block name be lowercase, but this was not enforced by the block parser
• The block parser enforces that a block name start with a letter, but this was not validated by registration

Further, these requirements were not documented in block registration documentation.

The proposed changes remedies each of the above issues.

Testing instructions:

Ensure tests pass:

npm test

Follow-up Tasks:

Gutenberg examples have names beginning with numbers and must be updated:

https://github.com/WordPress/gutenberg-examples

Codecov Report

Merging #3521 into master will increase coverage by 1.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3521      +/-   ##
==========================================
+ Coverage   34.54%   35.62%   +1.08%     
==========================================
  Files         261      262       +1     
  Lines        6710     6896     +186     
  Branches     1225     1286      +61     
==========================================
+ Hits         2318     2457     +139     
- Misses       3704     3738      +34     
- Partials      688      701      +13

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