Blocks: Efficiently find and traverse blocks in a document.

[dmsnell]

The existing block parser reliably parses blocks, but it's also a heavy operation, involving a full parse of the entire document, the construction of a full block tree which includes multiple copies of different spans of the source content, and the JSON-parsing of every span of block attributes.

In many situations it's only necessary to find specific blocks within a document, or find where they start or end, or build a map of document structure without needing the full parse.

WordPress should provide a reliable and efficient way to traverse the Blocks in an HTML document in a streaming manner.

Trac ticket: Core-61401

In this patch two new functions are introduced for the purpose of returning a PCRE pattern that can be used to quickly and efficiently find blocks within an HTML document without having to parse the entire document and without building a full block tree.

These new functions enable more efficient processing for work that only needs to examine document structure or know a few things about a document without knowing everything, including but not limited to:

• Finding the URL of the first image block in a document.
• Inserting hooked blocks.
• Analyzing block counts.