Add preference to enable/disable validation

[dhuebner]

It would be nice to have a possibility to disable XML validation

[fbricon]

You mean schema validation or also want to skip well-formedness checks?

[dhuebner]

@fbricon The best would be to have both. At the moment, the most important is schema validation

[angelozerr]

@dhuebner I'm aware to add an enableValidation preferences, but before I would like to understand why you need that?

[dhuebner]

@angelozerr Just my experience :)
There could be false positives or bugs in validation or you just expect a malformed xml (test cases).
E.g. if I run xml validator in eclipse for the org.eclipse.lsp4xml project it reports me over 120k errors