[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/tlschema/api/api.json

@@ -4322,8 +4322,9 @@
                   "text": "index: "
                 },
                 {
-                  "kind": "Content",
-                  "text": "string"
+                  "kind": "Reference",
+                  "text": "IndexKey",
+                  "canonicalReference": "@tldraw/utils!IndexKey:type"
                 },
                 {
                   "kind": "Content",
@@ -6064,8 +6065,9 @@
                   "text": "index: "
                 },
                 {
-                  "kind": "Content",
-                  "text": "string"
+                  "kind": "Reference",
+                  "text": "IndexKey",
+                  "canonicalReference": "@tldraw/utils!IndexKey:type"
                 },
                 {
                   "kind": "Content",
@@ -8299,8 +8301,9 @@
                   "text": "index: "
                 },
                 {
-                  "kind": "Content",
-                  "text": "string"
+                  "kind": "Reference",
+                  "text": "IndexKey",
+                  "canonicalReference": "@tldraw/utils!IndexKey:type"
                 },
                 {
                   "kind": "Content",