Improve type inference from AST

In the following PR https://github.com/alexprey/sveltedoc-parser/pull/71 we introduce improvement for variable type inference, but it is not support more complex cases. For example, all arrays are not generic and all functions don't have specification.

**input:**
```
const output = [1, 2, 3];
```
**expected output:**
```
type: "Array<number>"
```