@API(status = API.Status.STABLE)
    public <T> @Nullable T get(
            final @NonNull CommandFlag<T> flag
    ) {
        return this.getValue(flag).orElse(null);
    }
Returns a flag value.
<p>
If using {@link CommandFlag.FlagMode#SINGLE}
then this returns the only value, if it has been specified. If using
{@link CommandFlag.FlagMode#REPEATABLE} then
it'll return the first value.

@param flag Flag name
@param <T>  Value type
@return Stored value if present, else {@code null}
cloud-core/src/main/java/org/incendo/cloud/parser/flag/FlagContext.java