Add line-start and line-end to the symbol-placement options

[ClareFarley]

Motivation

There are currently three symbol placement options: point, line, and line-center.

It would be helpful to also have line-start and line-end as symbol-placement options. This could allow a user to style either end of a linestring with a symbol. For example, a specific symbol to mark the entrance of a tunnel and a different symbol to mark the departure from a tunnel.

Design Alternatives

The first point of the geometry is the start point, so a user can use point placement with a symbol layer that points to a source with line geometry. However, this only allows a user to style with the line-start, not the line-end.

[andrewharvey]

I guess for this to work features would need to be re-combined across tiles, otherwise at the edge of each tile you'll also see start/end points.

Another design alternative is require this as a pre-processing step where you add start/end points as separate features in your data. I used this approach in https://github.com/alantgeo/dataset-to-tileset which converted a Mapbox Dataset to a Mapbox Tileset but augmenting the data for doing arrowheads on lines.

[andrewharvey]

should we merge this with #4096, to me it's the same feature.

[l4j3b]

Would love to see that feature. Trying to draw "oriented" lines with mapbox draw and I would like to be able to signal the "start" or line (coordinates at index 0) with a point or so.

[JoshuaJMoore]

@ClareTrainor, @l4j3b If you guys could up vote this PR implementation on the actual PR for line-start and line-end that would be smashing #13638