Add TextFlag for encoding.TextMarshaler/TextUnmarshaler values

[ChrisJr404]

What type of PR is this?

• feature

What this does

Adds TextFlag, the cli equivalent of the standard library's flag.TextVar (added in Go 1.19). It lets any value implementing both encoding.TextMarshaler and encoding.TextUnmarshaler be populated directly from a flag.

The motivating case from #2051 is log-level configuration: *slog.LevelVar satisfies both interfaces, so you can wire the level straight into a flag and have the logger ready (with a sensible default) before parsing even finishes:

lv := &slog.LevelVar{} // defaults to INFO
cmd := &cli.Command{
    Flags: []cli.Flag{
        &cli.TextFlag{Name: "log-level", Value: lv},
    },
}
// --log-level WARN  ->  lv.Level() == slog.LevelWarn

Other stdlib types such as *netip.Addr and *time.Time work the same way, as do third-party types like zap's atomic level.

Implementation notes

• Follows the existing FlagBase[T, C, VC] value pattern, mirroring GenericFlag/StringFlag.
• Reuses StringConfig so TrimSpace is available.
• Adds a Command.Text(name) lookup, consistent with the other typed getters.
• Create only overwrites the target when a non-nil default Value is provided, so a Destination pointing at an existing target isn't clobbered by the nil interface default.

Testing

New flag_text_test.go covers set-from-arg, default value, Destination, TrimSpace, env source, invalid-value error propagation, and Command.Text. godoc-current.txt / testdata/godoc-v3.x.txt regenerated.

Closes #2051