Support type annotations

[davidbrochart]

Problem

Currently, a Doc or a shared type (Map, Array...) don't allow type annotations. This prevents catching errors at static analysis, or editor completion support.

Proposed Solution

Support type annotations on Doc and shared types. For instance:

from pycrdt import Array, Doc, Map

doc = Doc()
map0: Map[bool] = doc.get("map0", type=Map)
array0: Array[int] = doc.get("array0", type=Array)

map0["foo"] = "bar"  # error: Incompatible types in assignment (expression has type "str", target has type "bool")  [assignment]
array0.append("baz")  # error: Argument 1 to "append" of "Array" has incompatible type "str"; expected "int"  [arg-type]

Additional context

Some support for dynamic document validation was introduced in #38 (see also #65), but this issue is bout static typing.