Clean up naming of StringContent and FixedStringContent

It is suggested to rename as follows:

FixedStringContent -> Text
new FixedStringContent(String) -> Text.of(String)

StringContent -> TextBuilder

Also, most current uses of StringContent (specifically, those cases where it is immediately embedded in an enclosing tree node) can be replaced by FixedStringContent/Text. The primary exception is in ContentBuilder and HtmlTree, which provide `add(CharSequence)` methods, of which there are 157 usages.