Implement Syncback to support converting Roblox files to a Rojo project (#937)

This is a very large commit.
Consider checking the linked PR for more information.

src/project.rs

@@ -1,5 +1,5 @@
 use std::{
-    collections::{BTreeMap, HashMap, HashSet},
+    collections::{BTreeMap, HashSet},
     ffi::OsStr,
     fs, io,
     net::IpAddr,
@@ -7,11 +7,13 @@ use std::{
 };
 
 use memofs::Vfs;
-use rbx_dom_weak::{Ustr, UstrMap};
+use rbx_dom_weak::Ustr;
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
 
-use crate::{glob::Glob, json, resolution::UnresolvedValue, snapshot::SyncRule};
+use crate::{
+    glob::Glob, json, resolution::UnresolvedValue, snapshot::SyncRule, syncback::SyncbackRules,
+};
 
 /// Represents 'default' project names that act as `init` files
 pub static DEFAULT_PROJECT_NAMES: [&str; 2] = ["default.project.json", "default.project.jsonc"];
@@ -114,6 +116,10 @@ pub struct Project {
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub glob_ignore_paths: Vec<Glob>,
 
+    /// A list of rules for syncback with this project file.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub syncback_rules: Option<SyncbackRules>,
+
     /// A list of mappings of globs to syncing rules. If a file matches a glob,
     /// it will be 'transformed' into an Instance following the rule provided.
     /// Globs are relative to the folder the project file is in.
@@ -332,12 +338,21 @@ pub enum PathNode {
 }
 
 impl PathNode {
+    /// Returns the path of the `PathNode`, without regard for if it's optional
+    // or not.
+    #[inline]
     pub fn path(&self) -> &Path {
         match self {
             PathNode::Required(pathbuf) => pathbuf,
             PathNode::Optional(OptionalPathNode { optional }) => optional,
         }
     }
+
+    /// Returns whether this `PathNode` is optional or not.
+    #[inline]
+    pub fn is_optional(&self) -> bool {
+        matches!(self, PathNode::Optional(_))
+    }
 }
 
 /// Describes an instance and its descendants in a project.
@@ -367,16 +382,16 @@ pub struct ProjectNode {
     #[serde(
         rename = "$properties",
         default,
-        skip_serializing_if = "HashMap::is_empty"
+        skip_serializing_if = "BTreeMap::is_empty"
     )]
-    pub properties: UstrMap<UnresolvedValue>,
+    pub properties: BTreeMap<Ustr, UnresolvedValue>,
 
     #[serde(
         rename = "$attributes",
         default,
-        skip_serializing_if = "HashMap::is_empty"
+        skip_serializing_if = "BTreeMap::is_empty"
     )]
-    pub attributes: HashMap<String, UnresolvedValue>,
+    pub attributes: BTreeMap<String, UnresolvedValue>,
 
     /// Defines the behavior when Rojo encounters unknown instances in Roblox
     /// Studio during live sync. `$ignoreUnknownInstances` should be considered