Conform to `Sendable`

The types declared in the SwiftYMD module conform to Sendable now.

Fix calendars

• Fix all calendars used internally to be Gregorian

Initializers for type conversions

• YM(_ year: Int, _ month: Int)
• YMD(_ year: Int, _ month: Int, _ day: Int)
• YM(_ description: String)
• YMD(_ description: String)
• YM(_: Date, timeZone: TimeZone)
• YMD(_: Date, timeZone: TimeZone)
• Date(_: YM, timeZone: TimeZone)
• Date(_: YMD, timeZone: TimeZone)

Codable Year and Month

Year and Month conform to Codable.

YM and YMD do not conform to Codable because they have various encoded formats and it is preferable that users can specify the formats.

Fix typo

dayOfWeak to dayOfWeek

0.1.0-alpha

Enable Travis CI