fix: correct return type of getSchemaType

[mlewando]

I noted that the return type of getSchemaType is not correct. It looks to be derived from

export const SCHEMA_TYPES = ["string", "number", "integer", "boolean", "null", "array", "object"]

but the actual declaration was

export function getSchemaType(node: SchemaNode, data: unknown): keyof typeof SCHEMA_TYPES | undefined

The keyof typeof SCHEMA_TYPES is actually "length" | "push" | "pop" | "map" ... which is wrong.

I made the SCHEMA_TYPES a const array and created new simple type:

export type SchemaType = (typeof SCHEMA_TYPES)[number];

to fix that issue.