Adding the exponentiation operator for Double.

[Khalian]

Checklist

• [x] New Extension
• [x] New Test
• [ ] Changed more than one extension, but all changes are related
• [ ] Trivial change (doesn't require changelog)

[EZSwiftExtensionsBot]

	3 Messages
:book:	Executed 201 tests, with 0 failures (0 unexpected) in 6.294 (6.514) seconds
:book:	Executed 186 tests, with 0 failures (0 unexpected) in 5.521 (5.707) seconds
:book:	Executed 123 tests, with 0 failures (0 unexpected) in 4.246 (4.306) seconds

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