Printer: Also fully reconstruct whitespace in HTMLCloseTag

[marcoroth]

This pull request updates the IdentityPrinter to also reconstruct the whitespace in the HTMLCloseTag.

The parser was already adding the WhitespaceNodes to the children array of the HTMLCloseTag (done in #396), but the IdentityPrinter wasn't accounting for them.

Now these templates can also be fully reconstructed using the IdentityPrinter:

<div></   div>
<div></div   >
<div></   div   >

Additionally, this pull request also adds some more AST helpers to the @herb-tools/core package to select nodes before/after a certain Location or Position.