Improve type inference from AST

[alexprey]

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>"