IndexSignatureDeclarationStructure::keyType can't be a WriterFunction?

[ajvincent]

Describe the bug

Version: 20.0.0

ts-morph/packages/ts-morph/src/structures/interface/IndexSignatureDeclarationStructure.ts

Line 12 in a90dc91

keyType?: string;

This is a place where a type field can't be a WriterFunction. Most others (TypedNodeStructure, ReturnTypedNodeStructure, etc.) can.

To Reproduce

Not applicable. Without the type definition, I can't write code to set this.

TS-AST-Viewer link

[ajvincent]

@dsherret I want to take a shot at fixing this. But before I do so, I'd like to confirm with you that a fix is acceptable. I don't wish to break compatibility as you mentioned for PR #1412 .