Fix bug regarding multiline docstrings

If a multiline docstring has its closing quotes at the end of a line
instead of on a separate line, isort fails to properly add imports using
the `add_imports` config option; instead, it adds the desired imports
into the middle of the docstring as illustrated below. While PEP 257
(and other guides) advises that closing quotes appear on their own line,
`isort` should not fail here.

This change adds a check for closing docstrings at the end of a line in
addition to the existing line start check for all comment indicators. A
new section of the `test_add_imports` test explicitly tests multiline
imports and this failure scenario specifically.

---

A working example:

```python
"""My module.

Provides example functionality.
"""

print("hello, world")
```

Running `isort --add-import "from __future__ import annotations"`
produces the following as expected:

```python
"""My module.

Provides example functionality.
"""

from __future__ import annotations

print("hello, world")
```

---

The failure behavior described:

```python
"""My module.

Provides example functionality."""

print("hello, world")
```

Running `isort --add-import "from __future__ import annotations"` as
above produces the following result:

```python
"""My module.

from __future__ import annotations

Provides example functionality."""

print("hello, world")
```

Subsequent executions add more import lines into the docstring. This
behavior occurs even if the file already has the desired imports.