Add support for Dockerfiles that have multiple parser directives

[rcjsuen]

With the introduction of the syntax parser directive, Dockerfiles now technically support two parser directives:

• escape
• syntax

We need to:

1. deprecate Dockerfiles's getDirective() function as it only ever retrieves one directive
2. introduce a new getDirectives() function to Dockerfile that returns ParserDirective[]
3. update the parser to correctly populate getDirectives()

We should have supported getDirectives() from the beginning but did not bother to given that only the escape directive existed when the project was started. Now it is clear that would have been the better path to take.

At the Dockerfile's level, we have no idea whether someone is using the BuildKit backend or not. Although the BuildKit backend is considered to be experimental, given that the syntax directive is directly mentioned in the official channels I think we should support the parsing of it.