Enable examplePlaceholder without prefill to enable autocomplete compatibility

[johnfrancmartin]

• Backwards compatible with existing implementations
• Allows for compatibility of examplePlaceholder with iOS autocomplete, and for an alternate flow that doesn't enforce prefix (useful for dual-sim phones!)

Primary purpose is to enable examplePlaceholder and withPrefix to work with phone number autocomplete for textContentType = .telephoneNumber. iOS disables autocomplete if text is non-empty, so the current implementation is incompatible without this change.

To enable autocomplete when using withPrefix = true, withExamplePlaceholder = true, set withPrefixPrefill = false