Inferring type from constrained generics

[Muritavo]

The following pull request modifies how generics are handled by the docgen typing generation, for better support for generic enums when used with storybook.

Consider the following case:

function Component<T extends string>(props: { aGenericProp: T }) { return null }

The current implementation makes the generated type to be T that storybook resolves to any.
With this change it's checked if the typing is constrained, and if so, obtains the typing from the constraint.

So when it resolves it will see the component definition as:

function Component<T extends string>(props: { aGenericProp: string }) { return null }

These are the changes that have been made:

• GenericWithExtends.tsx: Exemplifies a use case and is used by the tests
• tests/parser.ts: This changes affects others tests that had the same structure, so some tests assertions were modified
• src/parser.ts: When generation the documentation, if the type has some constraint, generate the docs from the constraint