chore: enable Gitpod (#2335)

Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>

.gitpod.yml

@@ -0,0 +1,19 @@
+tasks:
+  - init: yarn install
+    command: yarn start --sockPort 443
+  - openMode: split-right
+    command: echo "Run 'yarn test' to run tests."
+
+ports:
+  - port: 7936
+    onOpen: open-preview
+
+github:
+  # https://www.gitpod.io/docs/prebuilds/#configure-prebuilds
+  prebuilds:
+    pullRequestsFromForks: true
+
+vscode:
+  extensions:
+    - dbaeumer.vscode-eslint@2.1.5:9Wg0Glx/TwD8ElFBg+FKcQ==
+    - flowtype.flow-for-vscode@1.5.0:AwOT6wgHTF43loZQCAUMLA==

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+    "javascript.validate.enable": false
+}

README.md

@@ -6,6 +6,7 @@
 [![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=KaTeX/KaTeX)](https://dependabot.com)
 [![jsDelivr](https://data.jsdelivr.com/v1/package/npm/katex/badge?style=rounded)](https://www.jsdelivr.com/package/npm/katex)
 ![](https://img.badgesize.io/KaTeX/KaTeX/v0.12.0/dist/katex.min.js?compression=gzip)
+[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/KaTeX/KaTeX)
 
 KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web.