Replace internal joined error with errors.Join

[sagikazarmark]

As preparation to Go 1.20 requirement, this PR backports some code from the Go stdlib and uses errors.Join to join multiple errors together.

It's a backward incompatible change in the sense that the error output becomes different:

• No more asterisks to denote error items (following errors.Join)
• No more error sorting

As an attempt to preserve some of the original behavior, Decoder.Decode continues to return errors in the following format (when multiple errors occur):

decoding failed due to the following error(s):

'Name' expected type 'string', got unconvertible type 'int', value: '123'
'Age' expected type 'int', got unconvertible type 'string', value: 'bad value'
'Emails[0]' expected type 'string', got unconvertible type 'int', value: '1'
'Emails[1]' expected type 'string', got unconvertible type 'int', value: '2'
'Emails[2]' expected type 'string', got unconvertible type 'int', value: '3'

This may change in the future