Allow skipWhitespaceOnlyText lexer option to be configurable

[shanekenney]

I have a use case where I need whitespace inside elements to be preserved when parsing an XML string. Currently whitespace only text nodes are skipped due to the hardcoded configuration supplied to the XMLStringLexer. Ideally I would like this to be configurable. This PR shows what I have in mind, but I would appreciate feedback if there's a solution that would be more appropriate.

Example

create(`<Element> </Element>`)

Currently this results in XML equivalent to the following with significant whitespace removed:

<Element></Element>

What I would like is to preserve the whitespace inside an element.

<Element> </Element>

I'm dealing with a rich text format where it's possible I'll encounter significant whitespace within elements and currently this is being lost.

Example

<Paragraph><Run format="bold">Hello</Run><Run> </Run><Run format="bold">world</Run></Paragraph>

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

[oozcitak]

Hey, thanks for that. Can you add some tests as well?

[shanekenney]

Need to prevent text nodes from being created within a document node.

[shanekenney]

@oozcitak I've added some tests. With skipWhitespaceOnlyText=false there was an issue creating text nodes within the document node so I'm skipping those cases. There's probably an argument to be made for only skipping whitespace text though? Let me know what you think.

[gapspt]

Thank you @shanekenney, I also need this functionality ^_^

It seems to be related to #121 (and #122). @oozcitak any chance on getting this merged? Thank you!