Fixes #9906

This pull request seeks to improve the block validation step to allow more leniency for effectively equivalent text encoded in varying forms.

The changes here were authored in such a way where there may be a slight performance benefit over master, both in a reduction of bundle size (an approximate 18% reduction gzipped on the blocks module) and in optimizing for an early return of equality if normalization (whitespace or encoding) is not necessary to determine equivalence of text sequences.

Implementation notes:

In the process of implementing further text normalization here, it was discovered that the underlying simple-html-tokenizer performs its own entities substitution when encountering text tokens in an HTML string. For the purposes of validation, this was considered to be redundant and was thus swapped with a stub entity parser in the included changes. Note that this is the change which enables the significant drop in bundle size. Note also as an aside that there's desire to consolidate to a single parser between the blocks parse and validator parse, so the use of simple-html-tokenizer may or may not persist far into the future.

Testing instructions:

Verify that block invalidation is not triggered by encoding variations.

For example, inserting the following HTML as the contents of a post (in Text Mode, Classic Editor Text tab, or directly in the database) should not be presented as an invalid block when next viewing the Visual Mode of the editor:

<!-- wp:paragraph -->
<p>This works. &#128517;</p>
<!-- /wp:paragraph -->

cc @MarkRH