Create EntityNotDeclared code action

[xorye]

Fixes #532

This PR adds a new code action to locally declare an undeclared entity.

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

Demo:

The codeaction works whether or not <!DOCTYPE is declared or not, and whether the internal subset [] exists or not.

Signed-off-by: David Kwon [email protected]

[fbricon]

please rebase against master

[xorye]

Thank you, fixed

[xorye]

Fixed, I also added a second commit that adds tests for #686, since that issue is being fixed thanks to this new method:
https://github.com/eclipse/lemminx/blob/765bc8fe1959f2f54bea9667e6c4b8e6320ea450/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/utils/XMLBuilder.java#L423-L432

[xorye]

Great feedback, fixed

[angelozerr]

@xorye you have 2 commits for this PR. I think the commit 084c611 should be removed, no?

[fbricon]

@angelozerr see #724 (comment)

[angelozerr]

@angelozerr see #724 (comment)

Thanks @fbricon

[angelozerr]

Great PR @xorye !