Make semantic model aware of docstring (#9960)
## Summary
This PR introduces a new semantic model flag `DOCSTRING` which suggests
that the model is currently in a module / class / function docstring.
This is the first step in eliminating the docstring detection state
machine which is prone to bugs as stated in #7595.
## Test Plan
~TODO: Is there a way to add a test case for this?~
I tested this using the following code snippet and adding a print
statement in the `string_like` analyzer to print if we're currently in a
docstring or not.
<details><summary>Test code snippet:</summary>
<p>
```python
"Docstring" ", still a docstring"
"Not a docstring"
def foo():
"Docstring"
"Not a docstring"
if foo:
"Not a docstring"
pass
class Foo:
"Docstring"
"Not a docstring"
foo: int
"Unofficial variable docstring"
def method():
"Docstring"
"Not a docstring"
pass
def bar():
"Not a docstring".strip()
def baz():
_something_else = 1
"""Not a docstring"""
```
</p>
</details>
