Split dpi module out into a separate crate

[madsmtm]

Part of #3433.

This PR is mostly about the documentation update, the actual API changes that Tauri needs can be done in subsequent PRs.

• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior

CC @amrbashir

[amrbashir]

Thank you, this is exactly what I was looking for. I will do a small PR for the single unit types afterwards.

[kchibisov]

what's the point to name it dpi and not winit-dpi, since it's tailored towards the dpi needs of winit?

[madsmtm]

Mostly because the name dpi was free, but also because the crate isn't really fundamentally tied to Winit.