Generate literal types for initialized constants

Have clutz generate out strict types for @const annotated fields.

For example:

```
/** @public {string} @const */
Foo.BAR = "baz";`
```

Should become:

```
static readonly BAR: 'baz'
```

in the generated declarations instead of the current behavior of:

```
static readonly BAR: string
```

This can help with type inferencing and the creation of string union types