(core) Improve encoding detection for csv imports, and make encoding an editable option.

Summary:
- Using a sample of data was causing poor detection if the sample were
  cut mid-character. Switch to using line-based detection.
- Add a simple option for changing encoding. No convenient UI is offered
  since config UI is auto-generated, but this at least makes it possible to
  recover from bad guesses.
- Upgrades chardet library for good measure.

- Also fixes python3-building step, to more reliably rebuild Python
  dependencies when requirements3.* files change.

Test Plan:
Added a python-side test case, and a browser test that encodings can
be switched, errors are displayed, and wrong encodings fail recoverably.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3979

app/plugin/FileParserAPI-ti.ts

@@ -15,6 +15,7 @@ export const ParseFileAPI = t.iface([], {
 export const ParseOptions = t.iface([], {
   "NUM_ROWS": t.opt("number"),
   "SCHEMA": t.opt(t.array("ParseOptionSchema")),
+  "WARNING": t.opt("string"),
 });
 
 export const ParseOptionSchema = t.iface([], {

app/plugin/FileParserAPI.ts

@@ -20,6 +20,7 @@ export interface ParseFileAPI {
 export interface ParseOptions {
   NUM_ROWS?: number;
   SCHEMA?: ParseOptionSchema[];
+  WARNING?: string;     // Only on response, includes a warning from parsing, if any.
 }
 
 /**