Add support for attribute docstring in the semantic model

[dhruvmanila]

Summary

This PR adds updates the semantic model to detect attribute docstring.

Refer to PEP 258 for the definition of an attribute docstring.

This PR doesn't add full support for it but only considers string literals as attribute docstring for the following cases:

1. A string literal following an assignment statement in the global scope.
2. A global class attribute

For an assignment statement, it's considered an attribute docstring only if the target expression is a name expression (x = 1). So, chained assignment, multiple assignment or unpacking, and starred expression, which are all valid in the target position, aren't considered here.

In __init__ method, an assignment to the self variable like self.x = 1 is also a candidate for an attribute docstring. This PR does not support this position.

Test Plan

I used the following source code along with a print statement to verify that the attribute docstring detection is correct.

Source code to test against:

a: int
"a: docstring"

b = 1
"b docstring" " continue"
"b: not a docstring"

c: int = 1
"c: docstring"

_a: int
"_a: docstring"

if True:
    d = 1
    "d: not a docstring"

(e := 1)
"e: not a docstring"

f = 0
f += 1
"f: not a docstring"

g.h = 1
"g.h: not a docstring"

(i) = 1
"i: docstring"

(j): int = 1
"j: docstring"

(k): int
"k: docstring"

l = m = 1
"l m: not a docstring"

n.a = n.b = n.c = 1
"n.*: not a docstring"

(o, p) = (1, 2)
"o p: not a docstring"

[q, r] = [1, 2]
"q r: not a docstring"

*s = 1
"s: not a docstring"


class Foo:
    a = 1
    "Foo.a: docstring"

    b: int
    "Foo.b: docstring"
    "Foo.b: not a docstring"

    c: int = 1
    "Foo.c: docstring"

    def __init__(self) -> None:
        self.x = 1
        "self.x: docstring"

        t = 2
        "t: not a docstring"

    def random(self):
        self.y = 2
        "self.y: not a docstring"

        u = 2
        "u: not a docstring"

    def add(self, y: int):
        self.x += y


def function():
    v = 2
    "v: not a docstring"


function.a = 1
"function.a: not a docstring"

I'll add this in the follow-up PR (#11302) which uses this method.

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