Enable formatter to skip instructions that span multiple lines

[rcjsuen]

Some users want to format instructions that span multiple lines in Dockerfiles themselves. The new setting added here will allow users to decide whether instructions that span multiple lines should be ignored by the formatter or not.

FROM scratch
RUN echo && \
            echo && \
                echo && \
        echo

1. Create the Dockerfile above.
2. Format the document. The lines in the RUN instruction will be formatted.
3. Undo the change so that it's back to its original form.
4. Open the settings.json file and add the following setting.

{
    "docker.languageserver.formatter.ignoreMultilineInstructions": true
}

5. Format the document again. The RUN instruction should not be formatted.

This fixes #992 and #2004.

[karolz-ms]

Thank you, Remy! 🤗

[rcjsuen]

@karolz-ms Thanks for the quick review. Have a good day!