(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

app/server/lib/GranularAccess.ts

@@ -87,6 +87,7 @@ const SPECIAL_ACTIONS = new Set(['InitNewDoc',
                                  'TransformAndFinishImport',
                                  'AddView',
                                  'CopyFromColumn',
+                                 'ConvertFromColumn',
                                  'AddHiddenColumn',
                                 ]);