Add support for referencing an external .swift-format

[rauhul]

Basically the title. I'd like to reference an externally vended and versioned swift-format config file.

For example these repos use the same config and currently have to copy it around:

• swift-mmio (canonical), swift-embedded-examples, and swift-matter-examples
• swift-syntax (canonical), swift-format, sourcekit-lsp

[MahdiBM]

That doesn't really align with the other stuff in the ecosystem to be referencing an external .swift-format.
For example on VS Code, sourcekit-lsp needs to be able to see the config file, otherwise it can't use it.

Another approach so solve this duplication problem might be the way hummingbird is handling things ...

[MahdiBM]

(that comment came too soon so here's the rest of it)

... which is to use a shared template repo and push updates to other repos when a file or something changes: hummingbird-project/hummingbird#608