[Snapping 1/5] Validation & strict types for fractional indexes (#2827)

Currently, we type our fractional index keys as `string` and don't have
any validation for them. I'm touching some of this code for my work on
line handles and wanted to change that:
- fractional indexes are now `IndexKey`s, not `string`s. `IndexKey`s
have a brand property so can't be used interchangeably with strings
(like our IDs)
- There's a new `T.indexKey` validator which we can use in our
validations to make sure we don't end up with nonsense keys.

This PR is part of a series - please don't merge it until the things
before it have landed!
1. #2827 (you are here)
2. #2831
3. #2793
4. #2841
5. #2845

### Change Type

- [x] `patch` — Bug fix

### Test Plan

1. Mostly relying on unit & end to end tests here - no user facing
changes.

- [x] Unit Tests

packages/validate/api/api.json

@@ -1543,6 +1543,43 @@
               },
               "implementsTokenRanges": []
             },
+            {
+              "kind": "Variable",
+              "canonicalReference": "@tldraw/validate!T.indexKey:var",
+              "docComment": "/**\n * Validates that a value is an IndexKey.\n *\n * @public\n */\n",
+              "excerptTokens": [
+                {
+                  "kind": "Content",
+                  "text": "indexKey: "
+                },
+                {
+                  "kind": "Reference",
+                  "text": "Validator",
+                  "canonicalReference": "@tldraw/validate!Validator:class"
+                },
+                {
+                  "kind": "Content",
+                  "text": "<"
+                },
+                {
+                  "kind": "Reference",
+                  "text": "IndexKey",
+                  "canonicalReference": "@tldraw/utils!IndexKey:type"
+                },
+                {
+                  "kind": "Content",
+                  "text": ">"
+                }
+              ],
+              "fileUrlPath": "packages/validate/src/lib/validation.ts",
+              "isReadonly": true,
+              "releaseTag": "Public",
+              "name": "indexKey",
+              "variableTypeTokenRange": {
+                "startIndex": 1,
+                "endIndex": 5
+              }
+            },
             {
               "kind": "Variable",
               "canonicalReference": "@tldraw/validate!T.integer:var",