Add random alias

[narthur]

This PR resolves #6.

Test instructions:

Open http://localhost:8080/random and refresh repeatedly

Changes:

• Adds .tool-versions file, pinning deno version
• Git-ignores .zed folder so I don't pollute the repo with my project-specific IDE config
• Corrects the return type of handler in index.ts
• Updates alias map to use anonymous functions as callbacks so that a new random emoji can be returned on each request
• Implements a random alias to return a new random emoji on each request
• Documents the new random alias on the home page