refactor: Reduce maximum block nesting in Liquid::BlockBody#parse

[dylanahsmith]

Problem

From #937 (comment)

CI is angry at me because of linter errors (I re-used BlockBody#parse's structure which has more than 3 levels of block nesting), the the rest of the test suite runs successfully.

Solution

Use early returns or next statements to reduce nesting. That way we no longer have a rubocop exception for this method.