CodeAction for EntityNotDeclared

[xorye]

(Originally from #518 )

Given this XML file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article [
  <!ELEMENT article (#PCDATA)>
]>          
<article>
	&nbsp;
</article>

  should be highlighted. (It should be managed in XMLSyntaxErrorCode).

CodeAction could create the proper entity in the DOCTYPE like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article [
  <!ELEMENT article (#PCDATA)>
  <!ENTITY nbsp "entity-value">
]>           
<article>
	&nbsp;
</article>

We must take care of existing of doctype. For instance if we have

<?xml version="1.0" encoding="UTF-8"?>          
<article>
	&nbsp;
</article>

CodeAction should generate

<!DOCTYPE article [
  <!ENTITY nbsp "entity-value">
]>

[xorye]

I think it would be best for me to handle this one since I have already started it.

[xorye]

This code action involves inserting indentation, so the natural idea is to get the insertSpaces boolean and tabSize integer from the XMLFormattingOptions from SharedSettings. The problem is that these values are only accurate during formatting requests.

When a formatting request is received, we create a new settings instance with the insertSpaces and tabSize values coming from the request here:
https://github.com/eclipse/lemminx/blob/034b2b3e2246f83765e5a8aa7626fe4c35127bbb/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/XMLTextDocumentService.java#L267

Unfortunately we do not get these values for code action requests. We need another way to get these values.

For VS Code, we cannot just read editor.tabSize and editor.insertSpaces from VS Code and map them to lemminx by creating new settings, because in VS Code you can set these values per file, from the bar at the bottom of VS Code:

Maybe we can deduce these values by reading the actual document itself?