Add new defs route for burwell line types

[aazaff]

Erika made a list of accepted new_types values for lines tables. This should be returned through the defs route. Same for the accepted points types.

The original list can be found at https://github.com/UW-Macrostrat/burwell-processing/blob/master/lines.md#lines

[cambro]

these really aren't "standard" definitions. If exposed in API (a good thing) it should be a select distinct x_type from table construct.