IndexSignatureDeclarationStructure::keyType can't be a WriterFunction?

**Describe the bug**

Version: 20.0.0

https://github.com/dsherret/ts-morph/blob/a90dc9118166ec484ea486a9d952c08d761dc6bc/packages/ts-morph/src/structures/interface/IndexSignatureDeclarationStructure.ts#L12

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](https://ts-ast-viewer.com/#code/C4TwDgpgBAglC8UDeAoKUDaBrCIBcUAzsAE4CWAdgOYC6BARgPaMA2EAhhSgL5A)

Start with packages/ts-morph/src/structures/interface/IndexSignatureDeclarationStructure.ts at the linked keyType definition, then compare it with the other type fields mentioned in the issue, such as TypedNodeStructure and ReturnTypedNodeStructure. Done means the keyType definition supports WriterFunction consistently with those fields and the relevant type checks pass.

Written by the indexing model from the issue text.