Define grammar for rule files

[zyxw59]

Currently, src/rssc.lalrpop just defines the syntax for regular expressions, but not syntax for:

• Rules
• Category definitions
• Token declarations
• Any other constructs that might be added

[zyxw59]

The syntax for the first two items should stay mostly the same as in previous versions to make transferring old files over as easy as possible:

• Rules: from > to / A_B ! C_D, where A and B refer to the environment of the change and C and D refer to the negated environment (i.e. environments that block the change)
• Category definitions: name = a b c, where name is the name of the category, and a b c are its elements.

That said, there's still some room for extensions to this syntax, such as rules with multiple environments joined with arbitrary logical operators, as well as new syntaxes that need to be defined, such as token declarations (a new feature for this version) and possibly features like macros.

[zyxw59]

Also, I'd like to move away from using LALRPOP (see #3), but the syntax it provides can be useful for defining the grammar.