Fix Struct and Interface Syntax Issues

[charlespascoe]

• Multiline comma not highlighted in structs e.g. the , isn't highlighted:

type Foo struct {
    A,
    B string
}

• Semicolons not highlighted in interfaces
• Embedded type with tag not highlighted as type, e.g. Bar isn't highlighted as a type:

type Foo struct {
    Bar `json:"bar"`
}