rubocops: add unless_multiple_conditions

[SeekingMeaning]

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Follow-up to #10245 (comment)

This PR adds the UnlessMultipleConditions rubocop, based on rubocop/rubocop#5400

• homebrew-core PR: various formulae: replace compound unless with if homebrew-core#68527
• linuxbrew-core PR: https://github.com/Homebrew/linuxbrew-core/pull/22057

Disallowed unless statements:

unless foo && bar
  something
end

something unless foo || bar

Truth table for reference:

p	q	!(p && q)	!p || !q	!(p || q)	!p && !q
F	F	T	T	T	T
F	T	T	T	F	F
T	F	T	T	F	F
T	T	F	F	F	F

[BrewTestBot]

Review period will end on 2021-01-08 at 23:12:39 UTC.

[MikeMcQuaid]

Great work here as usual @SeekingMeaning and impressive turnaround on this 👏🏻

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

@SeekingMeaning Looks like we may be able to replace this with rubocop/rubocop#9386 after the next RuboCop release? 🤞🏻

[SeekingMeaning]

Opened #10770