Merge std/dom_extensions into std/dom

[juancarlospaco]

It is literally a module with a single line, very very basic code,
I dunno why it was made that way, but I think it should be merged into std/dom.

• https://github.com/nim-lang/Nim/blob/devel/lib/js/dom_extensions.nim#L4
• https://nim-lang.github.io/Nim/dom.html

[timotheecour]

I dunno why it was made that way, but I think it should be merged into std/dom.

nim-lang/Nim#14979 (comment)

but I agree it should be merged in std/dom and std/dom_extensions should be deprecated; you can simply clarify in a doc comment that it may be experimental, if needed