This PR introduces two new keywords -- priority and exclusive -- for scene layers, to enable greater control over feature filter matching and styling composition.

Background
Tangram layer matching has always been inclusive, meaning a feature can match multiple layers (at multiple levels of the layer style tree), which are then merged to determine the final rendering parameters. Composing complex sub-layer trees can be used to capture both feature and zoom-dependent styling along orthogonal dimensions, for example varying road line widths and colors by separate criteria.

However, two in many ways simpler behaviors have not been easily expressed with the current layer syntax:

• The priority with which layers are matched has always been implicit. While layer depth takes priority (with an inheritance model), when layers at the same level match (siblings), the tie-breaker has been alphabetical, with the last lexically sorted layer name winning. This is cryptic at best and a significant hindrance to easy authoring where layer priority is important (see Bubble Wrap shield matching).
• Since multiple sibling layers will match by default, there has been no way to express simple exclusive relationships, for example an ordered list of filter conditions, where the first matching filter should be used, and the others discarded. While it has often been possible to work around this by carefully building filters to be functionally exclusive (for example, defining a filter A, then defining a filter B that negates all of A's conditions, in addition to including its own), this is also an unwieldy approach, and does not serve all cases (such as when the filters have natural overlaps that can't easily be separated in a way that avoids a feature from matching more than once).

New Syntax
The priority and exclusive keywords address these cases, and can be used both separately and together.

• priority: allows for the explicit definition of the matching order for sibling layers (alphabetical by layer name is still used as the default / fallback).
• exclusive: marking a layer as exclusive ensures that when that layer matches, no other sibling layers (and their cascading sub-layers, etc.) will; when multiple exclusive layers match, the higher priority layer wins.

For example, exclusive could express a single if-else filter condition:

layers:
  if:
    filter: ...
    exclusive: true
  else:
    ...

When used together, priority and exclusive can be used for chained if-elseif-else filter cases, to ensure that only one of an ordered set of conditions matches:

layers:
  layerA:                 # if matches layerA...
    filter: ...
    priority: 1
    exclusive: true
    draw: ...
  layerB:                 # else if matches layerB...
    filter: ...
    priority: 2
    exclusive: true
    draw: ...
  layerC:                 # else default to layerC...
    priority: 3
    exclusive: true
    draw: ...

Separate from "flow control" cases, priority can simply be used on its own to disambiguate the precedence of multiple layers without reliance on naming conventions.

Implementation
The implementation is a fairly narrow impact on the layer matching logic:

• When determining layer matching and merging order:
  • Layers with an explicit priority value are matched first, with values sorted from lowest to highest (e.g. priority: 1 is matched before priority: 2).
  • Layers without an explicit priority are matched after, and continue to be sorted lexically.
• Matching operation:
  • If a layer marked exclusive matches, all further matching at that level in the layer tree ceases and the matching layer is used (matching continues for that exclusive layer's child/descendant layers).
  • In the case that multiple layers are marked exclusive, the first one (according to the order described above) is used.