(core) Adding backend for 2-way references

Summary:
Adding support for 2-way references in data engine.

- Columns have an `reverseCol` field, which says "this is a reverse of the given column, update me when that one changes".
- At the time of setting `reverseCol`, we ensure that it's symmetrical to make a 2-way reference.
- Elsewhere we just implement syncing in one direction:
  - When `reverseCol` is present, user code is generated with a type like `grist.ReferenceList("Tasks", reverse_of="Assignee")`
  - On updating a ref column, we use `prepare_new_values()` method to generate corresponding updates to any column that's a reverse of it.
  - The `prepare_new_values()` approach is extended to support this.
  - We don't add (or remove) any mappings between rows, and rely on existing mappings (in a ref column's `_relation`) to create reverse updates.

NOTE This is polished version of https://phab.getgrist.com/D4307 with tests and 3 bug fixes
- Column transformation didn't work when transforming RefList to Ref, the reverse column became out of sync
- Tables with reverse columns couldn't be removed
- Setting json arrays to RefList didn't work if arrays contained other things besides ints
Those fixes are covered by new tests.

Test Plan: New tests

Reviewers: georgegevoian, paulfitz, dsagal

Reviewed By: georgegevoian, paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4322

test/server/lib/DocApi.ts

@@ -831,7 +831,8 @@ function testDocApi() {
             visibleCol: 0,
             rules: null,
             recalcWhen: 0,
-            recalcDeps: null
+            recalcDeps: null,
+            reverseCol: 0,
           }
         },
         {
@@ -852,7 +853,8 @@ function testDocApi() {
             visibleCol: 0,
             rules: null,
             recalcWhen: 0,
-            recalcDeps: null
+            recalcDeps: null,
+            reverseCol: 0,
           }
         },
         {
@@ -873,7 +875,8 @@ function testDocApi() {
             visibleCol: 0,
             rules: null,
             recalcWhen: 0,
-            recalcDeps: null
+            recalcDeps: null,
+            reverseCol: 0,
           }
         },
         {
@@ -894,7 +897,8 @@ function testDocApi() {
             visibleCol: 0,
             rules: null,
             recalcWhen: 0,
-            recalcDeps: null
+            recalcDeps: null,
+            reverseCol: 0,
           }
         },
         {
@@ -915,7 +919,8 @@ function testDocApi() {
             visibleCol: 0,
             rules: null,
             recalcWhen: 0,
-            recalcDeps: null
+            recalcDeps: null,
+            reverseCol: 0,
           }
         }
       ]
@@ -1715,7 +1720,8 @@ function testDocApi() {
               "visibleCol": 0,
               "rules": null,
               "recalcWhen": 0,
-              "recalcDeps": null
+              "recalcDeps": null,
+              reverseCol: 0,
             }
           }
         ]