Rename modes

[MichalBryxi]

This is a pet-peeve, but I still consider it a good DX. The modes in ember-highcharts seem to be arbitrary values that don't directly map to highcharts.com world. That being said, their product pages are not very consistent either. So since this is supposed to be consumed by developers, I'd go with whatever Highcharts.[api-call] is used in the source code:

mode option currently used	mode change proposal
falsy value	"chart"
"StockChart"	"stockChart"
"Map"	"mapChart"
"Gantt"	"ganttChart"

This would IMO make it easier to navigate for newcommers as they might be looking into the highcharts documentation and copy&pasting things.

Note that I wrote falsy -> "chart". This is not to remove the fallback value, but to support the effort to have an explicit chart mode: #486