Allow flagging of multiline implicit string concatenations

[charliermarsh]

At present, ISC001 and ISC002 flag concatenations like the following:

"a" "b"  # ISC001
"a" \
  "b"  # ISC002

However, multiline concatenations are allowed.

This PR adds a setting:

[tool.ruff.flake8-implicit-str-concat]
allow-multiline = false

Which extends ISC002 to also flag multiline concatenations, like:

(
  "a"  # ISC002
  "b"
)

Note that this is backwards compatible, as allow-multiline defaults to true.