Preserve all white space within foreignObject elements.

[johnkenny54]

This PR modifies SVGO to always preserve white space within <foreignObject> elements. In general there is no way to know the semantics of the <foreignObject> content, so all white space within the element is preserved.

The following changes were made:

• parser.js - keeps track of whether the parser is within a <foreignObject> element, and if so does not trim text or comment nodes.
• stringifier.js - keeps track of whether the stringifier is within a <foreignObject> element, and if so does not emit prettification indentation or newlines.
• _collections.js - removed 'pre' from textElems set - this was added by PR fix: dont trim pre elements #1796, but is not necessary since all <foreignObject> content now preserves white space.`.
• Added a test case to confirm that CDATA and comments are preserved within a <foreignObject>.
• Modified expected results in several test files. In all cases, the expected result is now to preserve the contents of the <foreignObject> element exactly.

Resolves #1728, resolves #1678, resolves #1473.