Replace signals implementation with TC39 signals

[gordonbrander]

Fixes #39

This PR:

• adds signal-polyfill as a dependency
• replaces our signals implementation with the native signals proposal polyfill
• introduces a vanilla browser bundle via esbuild

Background: It looks likely that reactive signals will become a language feature. TC39 signals are meant to be a lower-level primitive that is wrapped in a library. We're going to adopt signals by wrapping these low-level features in a higher-level API. The library API for Spellcaster will not change.

Using native signals has some significant advantages:

• Standard signals act as a Schelling point for the ecosystem.
  • Many libraries are contributing to the spec and/or adopting.
  • For example, Lit is implementing signal support, so adopting native signals would mean Spellcaster just works with Lit.
  • It seems plausible that future platform APIs may use them as well.
• All libraries that use TC39 signals under the hood will share the same signal graph, even if they expose different surface-level APIs.
• Spellcaster becomes even smaller.