Add rule to omit backticks unless required by the language

[calda]

Summary

Omit backticks around identifiers unless required by the language.

// WRONG
func travel(to planet: Planet, orbit: Orbit = .`default`) { ... }

// RIGHT
func travel(to planet: Planet, orbit: Orbit = .default) { ... }

Reasoning

Backticks are only required when escaping a keyword that would otherwise be invalid in that position. Many Swift keywords are contextual and are allowed as identifiers.