DevHub: Skip class-wp-block-parser.php from parsing

[SergeyBiryukov]

Background: ​#WP45309

Per ​https://core.trac.wordpress.org/ticket/45109#comment:8, WP_Block_Parser comes from an external package, and its @since tags are related to the package version, not the WordPress version.

At the moment, the class is ​parsed as a regular WP class, and is incorrectly shown in the reference as introduced in WordPress 3.8.0.

[coffee2code]

In 8324:

Developer: Skip class-wp-block-parser.php when parsing.

It is considered an external package.

Fixes #4144.
Props SergeyBiryukov.