Valid flag is for null, not validation

• UnmarshalText for JSON map keys #1
• Valid is set if the value is defined (not null), don't use it for e.g. form validation 8f0d579. Valid property is only for ft.N* types

Not updating major version to avoid this issue golang/go#58654. That is, importing "github.com/mozey/ft/v2" requires a change to go.mod. No downstream users depended on the property that was removed

Verify map key error messages

• Invalid ft.String, .Int, .Float, and .Bool error on MarshalText. Map keys must be valid
• Invalid values are marshalled to JSON as is, unlike ft.N types where the JSON value is null when the Valid flag is false
• Fixed a typo in the error message for ft.NBool.MarshalText
• Tests to verify map key error messages

Implement encoding.TextMarshaler

Custom types implement encoding.TextMarshaler (the MarshalText method). They can now be used as JSON map keys, see for example TestNMapKeys

Initial release

• Finalised v1 API (ignore v1.0.0 tag, oops)
• Consistent interface
• Text template tests