fix(typescript): Handle invalid type names

[sschw]

I recently saw that there is an issue with certain names in the schema.

components:
  responses:
    "400":
      description: Bad request

The schema above will generate the definition export type 400 = void where 400 is an invalid identifier.

The problem happens in schemaToTypeAliasDeclaration as the name is used without any check for its validity.

I addressed this issue by prepending invalid names by _ and if they are still invalid, by removing any non A-Za-z0-9 characters.

It might cause collisions though if you have e.g. 400 and _400, or you use a schema full of e.g. emojis as names.

[triglian]

+1