Lexical: Used non-breaking spaces instead of text spans for whitespace

Leading/trailing text whitespace would use spans with css whitespace
rules so the spaces would be represented, but this would lead to a
messier output.
Instead, this attempts to smartly use non-breaking spaces, along with
normal spaces, to represent white space in a way that still allows
breaking. It attempts to reduce non-breaking spaces where not needed
(next to other inline content).

Also fixes duplicate <em> usage on italic content.