Provide support string support to utcOffset method. Valid string values
formats for utcOffset method are +HH:mm, -HH:mm, +HHmm and -HHmm.

✅ Closes: iamkun#1085

Extend the current suite of utc unit tests by providing unit tests for
the utcOffset method with string arguments.

✅ Closes: iamkun#1085

UTC plugin constants should be self-contained from within the plugin
package.

✅ Closes: iamkun#1395