improve shouldExtractLiteralValuesFromEnum

[angular-moon]

More accurate determination of enumeration type and return the expected enumeration value

fixed: the following example does not generate an enumerated type

export enum Mode {
  A,
  B,
}

export enum Mode {
  A,
  B = "b",
}

fixed: return the expected enumeration value

export enum Mode {
 A,
 B = "b",
}

// old generated 
"type": {
   "name": "enum",
   "raw": "Mode",
   "value": [
     {
       "value": "Mode.A"
     },
     {
       "value": "\"b\""
     }
   ]
 }

// new generated 
"type": {
   "name": "enum",
   "raw": "Mode",
   "value": [
     {
       "value": "0"
     },
     {
       "value": "\"b\""
     }
   ]
 }

[angular-moon]

@pvasek Can you merge this PR?

[pvasek]

@angular-moon thanks a lot! Published on npm as v1.22.0.