feat: add errors.Is support to all errors

[gwvandesteeg]

Add functionality to support errors.Is on all generated errors to keep in line with best practice on checking whether an error is of the specified type as per changes to error handling in go1.13.

This allows code to go from

if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
  // do things
}

to

if !errors.Is(err, viper.ConfigFileNotFoundError{}) {
  // do things
}

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[gwvandesteeg]

This resolves #1736

[andig]

Imho errors.Is() already does this. The real discrepancy here is checking *Error vs Error.