Port syntax engine

[mtreca]

This is definitely more of a "someday" task than an immediate need.

After looking a bit closer at gnuplot-context, I realized that gnuplot.el provides its own context-aware completion engine by defining a subset of the gnuplot language grammar and uses it to parse data for completion. While this is impressive in its own right, I wonder if there are no built-in tools in Emacs that would allow to directly define the gnuplot grammar and would do the parsing for us, which would alleviate the maintenance cost of 2k+ lines of non-trivial elisp.

After looking for built-in solutions, semantic seems to be a viable option, and would allow nice integration of other built-in tools (eldoc and info come to mind, since this package manages these as well).

Now this is definitely not something I feel like I could implement easily myself (at least in the near future), but I would like to give it a try at some point to get better at elisp. If anyone else would like to give it a go, they are more than welcome to open a PR and/or contact me on the matter.