docs: Add ".ts" support in docs and configuration.

Add references to TypeScript in documentation where appropriate, such
as in example bash commands and discussions of the file structure.
Add a new section to the Reading List with TypeScript resources.
Also update `.editorconfig` to support ".ts" files.

Fix part of #12000.

docs/testing/linters.md

@@ -3,7 +3,7 @@
 ## Overview
 
 Zulip does extensive linting of much of its source code, including
-Python/JavaScript files, HTML templates (Django/handlebars), CSS files,
+Python/JavaScript/TypeScript files, HTML templates (Django/handlebars), CSS files,
 JSON fixtures, Markdown documents, puppet manifests, and shell scripts.
 
 For some files we simply check for small things like trailing whitespace,
@@ -92,7 +92,7 @@ Most of our lint checks get performed by `./tools/lint`.  These include the
 following checks:
 
 - Check Python code with pyflakes.
-- Check JavaScript code with eslint.
+- Check JavaScript and TypeScript code with eslint.
 - Check Python code for custom Zulip rules.
 - Check non-Python code for custom Zulip rules.
 - Check puppet manifests with the puppet validator.

docs/testing/testing-with-node.md

@@ -1,8 +1,8 @@
-# JavaScript unit tests
+# JavaScript/TypeScript unit tests
 
-Our node-based JavaScript unit tests system is the preferred way to
-test JavaScript code in Zulip.  We prefer it over the
-[Casper black-box whole-app testing](../testing/testing-with-casper.html),
+Our node-based unit tests system is the preferred way to test
+JavaScript/TypeScript code in Zulip.  We prefer it over the [Casper
+black-box whole-app testing](../testing/testing-with-casper.html),
 system since it is much (>100x) faster and also easier to do correctly
 than the Casper system.