Trait item completions could work better

[jonas-schievink]

Given this code:

trait Tr {
    type Ty;
    const KONST: u8;
    fn required();
    fn provided() {}
}

impl Tr for () {
    $0
}

The completion we provide are generally pretty bad:

• Requesting completions at $0 without typing anything else just shows generic completions. We should prioritize completions of missing trait items to be at the very top there.
  • (in fact I cannot find them in the list at all, do they only show up if at least one character is typed?)
• Replacing $0 with fn and then requesting completions shows nothing. Instead, both required and provided should show up.
  • (the same happens when typing other keywords)
• Applying the completion for Ty and KONST does not complete the required ; at the end, but the fn completions do include the function body and place the cursor inside.
• Impl items generated by macros are still suggested.

[Veykril]

Replacing $0 with fn and then requesting completions shows nothing. Instead, both required and provided should show up.

That's probably #11467