(core) New type conversion in the backend

Summary: This is https://phab.getgrist.com/D3205 plus some changes (https://github.com/dsagal/grist/compare/type-convert...type-convert-server?expand=1) that move the conversion process to the backend. A new user action ConvertFromColumn uses `call_external` so that the data engine can delegate back to ActiveDoc. Code for creating formatters and parsers is significantly refactored so that most of the logic is in `common` and can be used in different ways.

Test Plan: The original diff adds plenty of tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3240

test/nbrowser/gristUtils.ts

@@ -1053,6 +1053,10 @@ export async function setType(type: RegExp, options: {skipWait?: boolean} = {})
   if (!options.skipWait) { await waitForServer(); }
 }
 
+export async function applyTypeTransform() {
+  await driver.findContent('.type_transform_prompt button', /Apply/).click();
+}
+
 export async function isMac(): Promise<boolean> {
   return /Darwin|Mac|iPod|iPhone|iPad/i.test((await driver.getCapabilities()).get('platform'));
 }