Improve formatting of leading tokens in type union / intersection

[JohnnyMorganz]

#854 will introduce support for leading tokens in unions / interesctions, but we should format them better:

• If single line, remove a leading token
• If multi line, start with a leading token:

type Foo = 
    | X
    | Y

[JohnnyMorganz]

Prettier will probably be a good example for these cases. Interestingly, they only format leading for unions, not intersections: