adds check for `name` field's Ion type and occurence in type definition

[desaikd]

Description of changes: This PR works on issues related to name field in a type definition as mentioned in https://github.com/amzn/ion-schema-rust/pull/124#discussion_r1023352762.

Issue: Type definitions described as below with multiple name fields and of non symbol type should return an error:

// invalid type definition with multiple names
type:: {
   name: my_type,
   name: new_type
}

// invalid type definition which has `name` field with non symbol type
type:: {
     name: "my_type"
}

List of changes:

• adds check for multiple name fields in a type definition
• adds check for non symbol name field values
• modifies doc tests to not accept string as type name

Tests

• adds unit tests for both occurrence and type of name field in type definition