Parsing ACSetSpecs from files with PEG.jl

A good test case for using PEG to understand how to connect DSLs specified with ADTs and surface syntax specified with PEG grammars would be to give a surface syntax for ADTs that are used for the acsetlang. This would let us read ACSets from files and extract an ADT of the acset language and then `construct` instances of the ACSets described in the language.

ADT definition and Usage:
https://github.com/AlgebraicJulia/ACSets.jl/blob/0c5b06649ea6678d2f5668559d8c933f971b62b6/src/ADTs.jl#L148
https://github.com/AlgebraicJulia/ACSets.jl/blob/0c5b06649ea6678d2f5668559d8c933f971b62b6/test/ADTs.jl#L70

Example PEG rules for UWDs:
https://github.com/AlgebraicJulia/SyntacticModels.jl/pull/11

Exit criteria:
1. read ACSets from files and extract an ADT of the acset language
2. `construct` instances of the ACSets described in the language
3. primitive rules like whitespace and parenthesis parsing is in a reusable module