UDT field completions

[minestarks]

Builds on #1947 .

Enables:

1. Field access completions, for example:

let x = Complex(1.0, 2.0);
x.| 

At the cursor location indicated by |, completions should include Real since it's a field of Complex.

2. Field assignment completions, for example:

struct Foo {
    bar: Int
}

new Foo { | }

Here, completions should include Bar.

[sezna]

really appreciate the extra documentation and tests