Add new experimental rule function-spacing

[paul-dingemans]

Lints and formats the spacing after the fun keyword.

Example:

            // Too many space
            fun  foo() = "some-result"
            
            // Unexpected newline
            fun
            foo() = "some-result"

This rule is required to create a rule which can rewrite the function signature automatically as is described in #1341

Checklist

• tests are added
• CHANGELOG.md is updated