(core) Speed up and upgrade build.

Summary:
- Upgrades to build-related packages:
  - Upgrade typescript, related libraries and typings.
  - Upgrade webpack, eslint; add tsc-watch, node-dev, eslint_d.

- Build organization changes:
  - Build webpack from original typescript, transpiling only; with errors still
    reported by a background tsc watching process.

- Typescript-related changes:
  - Reduce imports of AWS dependencies (very noticeable speedup)
  - Avoid auto-loading global @types
  - Client code is now built with isolatedModules flag (for safe transpilation)
  - Use allowJs to avoid copying JS files manually.

- Linting changes
  - Enhance Arcanist ESLintLinter to run before/after commands, and set up to use eslint_d
  - Update eslint config, and include .eslintignore to avoid linting generated files.
  - Include a bunch of eslint-prompted and eslint-generated fixes
  - Add no-unused-expression rule to eslint, and fix a few warnings about it

- Other items:
  - Refactor cssInput to avoid circular dependency
  - Remove a bit of unused code, libraries, dependencies

Test Plan: No behavior changes, all existing tests pass. There are 30 tests fewer reported because `test_gpath.py` was removed (it's been unused for years)

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3498

app/common/Locales.ts

@@ -19,7 +19,7 @@ try {
   const display = (code: string) => {
     try {
       const locale = new Intl.Locale(code);
-      const regionName = regionDisplay.of(locale.region);
+      const regionName = regionDisplay.of(locale.region!);
       const languageName = languageDisplay.of(locale.language);
       return `${regionName} (${languageName})`;
     } catch (ex) {
@@ -58,7 +58,7 @@ export function getCurrency(code: string) {
 try {
   const currencyDisplay = new Intl.DisplayNames('en', {type: 'currency'});
   currencies = [...new Set(currenciesCodes)].map(code => {
-    return {name: currencyDisplay.of(code), code};
+    return {name: currencyDisplay.of(code)!, code};
   });
 } catch {
   // Fall back to using the currency code as the display name.