Revisit text_block_heading rule to fix issues

[shunguoy]

This rule applies to the DOM elements p, div, and br. The issues are:
(1) The br is usually just a line break without any child (Content model: Nothing), such as
or
.
(2) other elements should be considered: strong, em etc.
(3) the test case results are wrong, such as test/v2/checker/accessibility/rules/text_block_heading_ruleunit/Headings-noneUsedEmphasizedText.html. which shouldn't report errors in
elements that have no child.

[shunguoy]

Triage: we are looking at the block level elements only with new lines, rather than individual text sections.