internal: Add SyntaxFactory to ease generating nodes with syntax mappings

[DropDemBits]

Part of #​15710

Instead of requiring passing a &mut SyntaxEditor to every make constructor to generate mappings, we instead wrap that logic in SyntaxFactory, and afterwards add all the mappings to the SyntaxEditor.

Includes an example of using SyntaxEditor & SyntaxFactory in the extract_variable assist.

[Veykril]

@bors r+

[bors]

📌 Commit f9ad9a0 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit f9ad9a0 with merge 233b1ac...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 233b1ac to master...

[Veykril]

Not really the place to discuss but I wonder how this could play into having quasi quoting #2227. I guess that could take a factory as an extra input or return something that needs to go through the factory to be "registered"

[DropDemBits]

@Veykril Having an extra input to the quasi quoting macro(s) would probably be best, as doing something like post-registering with the factory would mean that we'd always need to generate SyntaxMappings, even if we don't need it.