Create new API for retrieving a flag's options

[rcjsuen]

The experimental Dockerfile frontends have exposed some new flags for RUN instructions.

# syntax = docker/dockerfile:experimental
FROM ubuntu
RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
RUN --mount=type=cache,target=/var/cache/apt --mount=type=cache,target=/var/lib/apt \
  apt update && apt-get --no-install-recommends install -y gcc

We should expose an API on Flag so that we can retrieve the options and their corresponding values. We can do this by exposing a new FlagOption type that can be retrieved from Flag.

interface FlagOption {
    public getName(): string;
    public getValue(): string | null;
    public getNameRange(): Range;
    public getValueRange(): Range | null;
    public getRange(): Range;
}

interface Flag {

    public getOption(name: string): FlagOption | null;

    public getOptions(): FlagOption[];

}