Refactor to use layer name "constants" rather literal values?

[follower]

For clarity, it would be nice to have layer names rather than layer numbers (e.g. layers: [ 21, 25, 29, 51 ],) in the code.

The mixed-case nature of the standard Eagle layer names is unfortunate and it's not immediately clear what constant naming convention might be best (especially given JS's support/lack "const" and enums).

Maybe one of these styles:

// tDocu
EAGLE_LAYER_tDocu
LAYER_tDocu
EAGLE_LAYER_TDOCU
LAYER_TDOCU
layer.tDocu
layer.tDocu.id

Or something...