chore: modernize Deno setup

[justjavac]

Summary:

• Modernize UserError as a native Error subclass with ErrorOptions support.
• Add deno.json tasks/imports and deno.lock for JSR @std/assert.
• Update CI to Deno v2.x and current GitHub actions.
• Refresh README and VS Code Deno extension recommendation.

Tests:

• deno task check