Added parsing of PHPDoc blocks based on AST and PHPDoc parser by phpstan.

[zinigor]

Proposed changes:

• Makes the doc parser that hasn't previously been used switch to PHP's AST extension with help from phpstan/phpdoc-parser.
• Adds detection of apply_filters calls using AST and PHPDoc parsing using phpdoc-parser.

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Checkout this branch.
• Run jetpack docs
• Observe the phpdoc.json file generated in the folder where you're running the command from.

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[zinigor]

@since and @module tags will have to be added separately.

[tbradsha]

I can confirm it generates both a phpdoc.json and a markdown.json. I'm not familiar enough with AST parsing to be confident as to whether it's doing what it should at a low level, though.

I'll note that there are two deprecation notices from the scribu library, and several instances of this:

Warning: DOMDocument::loadHTML(): Tag summary invalid in Entity, line: 376 in /some/path/jetpack/tools/cli/helpers/doc-parser/runner.php on line 72