(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/server/lib/UnsafeNodeComponent.ts

@@ -121,8 +121,8 @@ export class UnsafeNodeComponent extends BaseComponent {
     .catch(err => log.warn("unsafeNode[%s] failed with %s", child.pid, err))
     .then(() => { this._child = undefined; });
 
-    child.stdout.on('data', makeLinePrefixer('PLUGIN stdout: '));
-    child.stderr.on('data', makeLinePrefixer('PLUGIN stderr: '));
+    child.stdout!.on('data', makeLinePrefixer('PLUGIN stdout: '));
+    child.stderr!.on('data', makeLinePrefixer('PLUGIN stderr: '));
 
     warnIfNotReady(this._rpc, 3000, "Plugin isn't ready; be sure to call grist.ready() from plugin");
     child.on('message', this._rpc.receiveMessage.bind(this._rpc));