feat: Add traversal methods to Element

[KevinYeramian]

What does this PR do?

• Add traversal methods to Element

I used the word traversal because it used in the official documentation to refers to these methods (link)

[karfau]

The method _insertBefore is calling the remove method but has extra code for joining inserting a child into the NodeList, can you please check if something also needs to be added there?

[karfau]

We also need some documentation:

• listing them in the readme as part of the section of the spec that first introduced them.
• jsdoc comments on the properties with minimal description taken from the spec, pointing to any differences from the spec.
  And ideally I would prefer to mark those properties as @readonly in a "tsdoc" comment way (you should be able to find other examples in the code base), to get a bit closer to having getters instead of normal properties.

[karfau]

Thank you a lot for the contribution so far.
I provided some feedback that needs to be addressed, but it seems to be a much smaller effort than I originally anticipated.

[KevinYeramian]

Review fixes committed!

• Not sure what to check for your comment
• I added the "tsdoc" but it current format seems little bit “heavy" in the code.

[karfau]

Review fixes committed!

Nice, loving it so far

• Not sure what to check for your comment

Yeah, that comment is a bit hard to grasp, sorry.
I think we are missing updating the new props as part of the Node.insertBefore API, which is also used as part of other methods:
https://github.com/brodybits/xmldom/blob/c234c4d08b482841886b703547dd62697e6d863d/lib/dom.js#L448-L462

• I added the "tsdoc" but it current format seems little bit “heavy" in the code.

Looks great for me.

[karfau]

The method _insertBefore is calling the remove method but has extra code for joining inserting a child into the NodeList, can you please check if something also needs to be added there?

@KevinYeramian This comment points to my main concern for this PR so far. Do you still have capacity to continue working on it? I can help to relsove the conflicts if that's an issue for you.

[karfau]

I resolved the commit, which leads to failing tests (due to the breaking changes in version 9.
I will see if I find time to get back to it, but it just didn't feel right to close the PR.