Rule request: Avoid Yoda conditions

[marcelofabri]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

In Swift, you need a var or let keyword to do an assignment inside a condition (if, guard, etc).
Thus, you only get the disadvantages of Yoda conditions, but no benefits.

2. Provide several examples of what would and wouldn't trigger violations.

// should trigger
if 42 == foo {}
guard "str" == bar else { return }
while 10 == baz { }

// shouldn't trigger
if foo == 42 {}
guard bar == "str" else { return }
while baz == 10 { }
if foo == bar {}

3. Should the rule be configurable, if so what parameters should be configurable?

Just the severity.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a
   rule as opt-in.

I think this might cause some false positives, so I'd say opt-in. But it could be enabled by default if we can avoid those false positives.

[jpsim]

Good rule, but definitely opt-in.

[jpsim]

Done in #1965