At present, `ISC001` and `ISC002` flag concatenations like the following:
```py
"a" "b" # ISC001
"a" \
"b" # ISC002
```
However, multiline concatenations are allowed.
This PR adds a setting:
```toml
[tool.ruff.flake8-implicit-str-concat]
allow-multiline = false
```
Which extends `ISC002` to _also_ flag multiline concatenations, like:
```py
(
"a" # ISC002
"b"
)
```
Note that this is backwards compatible, as `allow-multiline` defaults to `true`.
This _did_ fix https://github.com/charliermarsh/ruff/issues/1894, but was a little premature. `toml` doesn't actually depend on `toml-edit` yet, and `v0.5.11` was mostly about deprecations AFAICT. So upgrading might solve that issue, but could introduce other incompatibilities, and I'd like to minimize churn. I expect that `toml` will have a new release soon, so we can revert this revert.
Reverts charliermarsh/ruff#2040.
More accurate since the enum also encompasses:
* ALL (which isn't a prefix at all)
* fully-qualified rule codes (which aren't prefixes unless you say
they're a prefix to the empty string but that's not intuitive)
