fix essential parsing.

[skewballfox]

In order to have snippets composed of snippets, without adding anything else, it is necessary to parse tabstops and subsnippets, and store their correct locations.

Since this is a rather vague problem, composed of a lot of smaller but related specific problems, it's probably best to just make this into a list of issues.

here are the problems right now:

• need to append \n to the end of end text sections, OR need to change how snippets are stored so that they are a single string rather than a vector of strings
• need to handle component creation directly from the parser
• need to actually handle variables/Functors
• need to handle the transforms of variables

this list will be updated as I make progress or discover new issues.

[skewballfox]

The latest commit fixed the missing newlines by changing how snippets are stored. Since add_target and drop_target return nothing to the caller, it makes sense to move more work to that step when possible. also since the only time the dashmap containing the snippets needs to be accessed is when processing a get_snippet request, now it only contains the content, and all other relevant information to a snippet has been moved into SnippetSet