Add high-level theory/guiding principles

Most people don't have the time or energy to read through the entire style guide and internalize it. Additionally, there are always going to be gaps that we haven't explicitly handled.

For both of these cases, I think it would be nice to add a few concise guiding principles to the document. This would allow people to essentially "memorize these 6 ideas and you will know what to do 99% of the time."

I don't know what these principles are or should be, but I figured I would get the discussion started here.

Start by reviewing the existing JavaScript style guide, since the issue does not identify a file, section, or test. Determine the proposed concise guiding principles through project discussion, then document the agreed principles in the style guide; done means the scope and wording have been accepted and published.

Written by the indexing model from the issue text.