Fix crash for interpolation in pattern key

[thomasmarshall]

case 1
in {"a:#{1}": a} => i
 puts "a: #{a} and i: #{i}"
end

The symbol literal with interpolation is not allowed when pattern matching here. Prism raises an error but still parses the tree like so:

@ AssocNode (location: (2,4)-(2,13))
├── flags: ∅
├── key:
│   @ InterpolatedSymbolNode (location: (2,4)-(2,13))
│   ├── flags: ∅
│   ├── opening_loc: (2,4)-(2,5) = "\""
│   ├── parts: (length: 2)
│   │   ├── @ StringNode (location: (2,5)-(2,7))
│   │   │   ├── flags: static_literal, frozen
│   │   │   ├── opening_loc: ∅
│   │   │   ├── content_loc: (2,5)-(2,7) = "a:"
│   │   │   ├── closing_loc: ∅
│   │   │   └── unescaped: "a:"
│   │   └── @ EmbeddedStatementsNode (location: (2,7)-(2,11))
│   │       ├── flags: ∅
│   │       ├── opening_loc: (2,7)-(2,9) = "\#{"
│   │       ├── statements:
│   │       │   @ StatementsNode (location: (2,9)-(2,10))
│   │       │   ├── flags: ∅
│   │       │   └── body: (length: 1)
│   │       │       └── @ IntegerNode (location: (2,9)-(2,10))
│   │       │           ├── flags: static_literal, decimal
│   │       │           └── value: 1
│   │       └── closing_loc: (2,10)-(2,11) = "}"
│   └── closing_loc: (2,11)-(2,13) = "\":"
├── value:
│   @ MissingNode (location: (2,4)-(2,13))
│   └── flags: ∅
└── operator_loc: ∅

Valid node types for the key are specified here.