Add Polar chart

[tmecklem]

This PR adds the polar chart type to the chart library, where a polar chart is defined by a chart with polar axes r and t. r represents the radius distance of a point on the chart, and t represents the angular offset from 0 in radians. It also adds the polar chart LiveView Component to the library.

To Do (uncharted core):

• [x] Add polar coordinate system (Chart, Dataset, Axes)
• [ ] Add degree units to radians conversion by specifying units: :degrees in axis

To Do (uncharted_phoenix):

• [x] Render points
• [x] Render labels for points
• [x] Render t axis (angular) gridlines
• [ ] Render r axis (concentric circles) gridlines