[Feature Request] DateAdapter: support formatByString function

[mtdvlpr]

Problem to solve

I want to use the date adapter to format a date with a custom string format. Currently you have to use the format function with predefined options.

Proposed solution

Implement the @date-io formatByString function:
formatByString(value: TDate, formatString: string): string;

[KaelWD]

This is not supported by Intl.DateTimeFormat, use a more complete library like date-fns for custom formatting.

[mtdvlpr]

@KaelWD, I am using the @date-io/date-fns adapter in the Vuetify config, so I expect the function that it supports, to be supported in the useDate function. Now that I try it in Vuetify Playground, it does work, but TypeScript says the function doesn't exist. So I guess this is more of a Bug Report...

Should I make a separate issue for this?

[KaelWD]

Hmm yeah we probably need some way to configure the types if you're using a full adapter.