x/tools/gopls: explore UX of fillstruct for partially populated nested structs

[jeanbza]

Continuation of #39804, in which in cl262018 we introduce basic support for partially filled structs: how should this work for nested structs? Should all levels of the nested structs populate? Or only one level? If one level, which level, when the cursor is between two of the nested struct levels, like,

f := Foo{
  Bar: {}, // cursor is on the closing bracket: fill Foo, or Bar, or both?
}