Rewrite the document symbols code to create an AST

[rcjsuen]

At the moment, every response handler parses a Dockerfile with its own unique code. While a Dockerfile's syntax is not exactly very complicated, a lot of code that is very similar is replicated across the entire codebase. We should rewrite a parser from scratch that will create an AST of sorts that knows where a comment, directive, and instruction line starts and ends (including any escaped newlines in the case of instructions).

To validate that this parser is working as intended, it will first be used to replace dockerSymbols.ts before further refactorings across the entire codebase.