CodeAction to activate `xml.validation.resolveExternalEntities` to true

By default xml.validation.resolveExternalEntities is set to false for security issue. It is nice but it can be hard for user to know why XML validation / completion based on DTD doesn't work when xml.validation.resolveExternalEntities is set to false.

I encountered the problem and I though it was a bug from LemMinx, but after debugging LemMinx I noticed that it was just a problem with xml.validation.resolveExternalEntities which should be set to true.

Here my usecase.

• my xml.validation.resolveExternalEntities was set to false.
• I open vscode
• I created a SVG file like this:

<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="5cm" height="4cm" version="1.1"
	xmlns="http://www.w3.org/2000/svg">
</svg>

• LemMinx download the DTD and after that I see those error:

• at this step, I had the impression that LemMinx was bugged, but the fix is to set xml.validation.resolveExternalEntities to true
• but even if you swith this setting to true, you have again the error. You have validation error and completion based on DTD doesn't work.
• you need to close vscode and reopen it. And now validation and completion based on DTD is working:

We need really improve that:

• provide a code action which set xml.validation.resolveExternalEntities to true
• the validation and completion based on DTD should work when this setting is set to true (it is a matter of cache that it should be evicted). See https://github.com/eclipse/lemminx/issues/1123

Start by tracing how the vscode-xml extension handles xml.validation.resolveExternalEntities and how LemMinx caches DTD resolution; the issue links LemMinx issue 1123 as related context. Done means a code action can enable the setting and validation and DTD completion update without restarting VS Code.