Allow severity of duplicate_imports rule to be configurable

[SimplyDanny]

Fixes #5978.

[SwiftLintBot]

	18 Messages
📖	Building this branch resulted in a binary size of 29488.84 KiB vs 29488.71 KiB when built on main (0% larger).
📖	Linting Aerial with this PR took 0.17 s vs 0.17 s on main (0% slower).
📖	Linting Alamofire with this PR took 0.17 s vs 0.17 s on main (0% slower).
📖	Linting Brave with this PR took 0.78 s vs 0.76 s on main (2% slower).
📖	Linting DuckDuckGo with this PR took 0.54 s vs 0.54 s on main (0% slower).
📖	Linting Firefox with this PR took 1.01 s vs 1.0 s on main (1% slower).
📖	Linting Kickstarter with this PR took 0.78 s vs 0.81 s on main (3% faster).
📖	Linting Moya with this PR took 0.13 s vs 0.13 s on main (0% slower).
📖	Linting NetNewsWire with this PR took 0.31 s vs 0.31 s on main (0% slower).
📖	Linting Nimble with this PR took 0.15 s vs 0.15 s on main (0% slower).
📖	Linting PocketCasts with this PR took 0.71 s vs 0.73 s on main (2% faster).
📖	Linting Quick with this PR took 0.12 s vs 0.12 s on main (0% slower).
📖	Linting Realm with this PR took 0.43 s vs 0.42 s on main (2% slower).
📖	Linting Sourcery with this PR took 0.28 s vs 0.28 s on main (0% slower).
📖	Linting Swift with this PR took 0.42 s vs 0.41 s on main (2% slower).
📖	Linting VLC with this PR took 0.2 s vs 0.2 s on main (0% slower).
📖	Linting Wire with this PR took 1.72 s vs 1.75 s on main (1% faster).
📖	Linting WordPress with this PR took 1.06 s vs 1.04 s on main (1% slower).

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