Add a new syncRules field project files to allow users to specify middleware to use for files

[Dekkonot]

This PR adds the ability to specify a snapshot middleware to use for files that match a given glob, essentially allowing user-customization of Rojo's snapshotting system. For better UX, the name used for this feature is Sync Rules since it aligns with the concepts we use elsewhere (Transformer Rules would also work but I worry that it would imply we were modifying the contents of files), rather than anything mentioning middleware or snapshotting.

An explanation of the syntax is provided in the changelog.

The bulk of this implementation is the process of generalizing the snapshot middleware to not rely upon file names so heavily. This behavior is still present in init files, since the semantics of changing sync rules for those is unclear (should it just be every file named init if it matches a middleware rule?). A suffix field is included to support multiple file extensions like Rojo uses, which means that everything we currently do (minus .meta.json files) is representable as a syncing rule.

My motivation for this feature is two-fold:

1. A more rough implementation of this is used at Uplift Games to sync our wally.lock and git ref information into Adopt Me for the sake of build integrity and identification. It's a blocker for us updating Rojo, and I would like to get it added here so that any subsequent changes I make at Uplift can be pushed directly to Rojo rather than to our fork.

2. The existence of syncing rules makes it more practical to change file extensions or support per-script RunContext adjustments since there's a way to maintain compatibility now. It may be worth introducing new variants of the Middleware enum for RunContext for scripts to support users specifying their own extensions to support it.

I'm not 100% on the names of the middleware, but I'm open to feedback on them.

[kennethloeffler]

Looking pretty ok - I need to go feed myself right now, but I'll be back shortly for a closer look at snapshot_middleware/mod.rs. A few comments:

[Corecii]

(no action needed, just a comment)

Similar to Ken's comment later in this file, I'm thinking about if there's a better way to do this.

It would be cool if we could support user-defined init types, but that seems like a bit of a mess. It would let us move all of this logic to default sync rules, though! I wonder if there's a way we can ergonomically define priorities for these things with a future version of sync rules, such that we just have internal default sync rules that maintain this priority?

[Corecii]

Two potential minor improvements that aren't blockers -- fix them or leave them up to you!

[chriscerie]

Overall looks good

[Corecii]

still looks good to me!