(core) Fix imports into reference columns, and support two ways to import Numeric as a reference.

Summary:
- When importing into a Ref column, use lookupOne() formula for correct previews.
- When selecting columns to import into a Ref column, now a Numeric column like
  'Order' will produce two options: "Order" and "Order (as row ID)".
- Fixes exports to correct the formatting of visible columns. This addresses multiple bugs:
  1. Formatting wasn't used, e.g. a Ref showing a custom-formatted date was still presented as YYYY-MM-DD in CSVs.
  2. Ref showing a Numeric column was formatted as if a row ID (e.g. `Table1[1.5]`), which is very wrong.
- If importing into a table that doesn't have a primary view, don't switch page after import.

Refactorings:
- Generalize GenImporterView to be usable in more cases; removed near-duplicated logic from node side
- Some other refactoring in importing code.
- Fix field/column option selection in ValueParser
- Add NUM() helper to turn integer-valued floats into ints, useful for "as row ID" lookups.

Test Plan: Added test cases for imports into reference columns, updated Exports test fixtures.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3875

test/fixtures/export-csv/reference.csv

@@ -1,4 +1,4 @@
-Text Bar,Int Text,Text Formula
-"the ""quote marks"" ?",Integer[500],a --- grist https://www.getgrist.com/
-"b ,d",Integer[200],"b ,d --- https://www.getgrist.com/"
-"the ""quote marks"" ?",,a --- grist https://www.getgrist.com/
+Text Bar,Int Text,Text Formula,RowId
+"the ""quote marks"" ?",500,a --- grist https://www.getgrist.com/,Text[3]
+"b ,d",200,"b ,d --- https://www.getgrist.com/",Text[2]
+"the ""quote marks"" ?",0,a --- grist https://www.getgrist.com/,