deprecate `lightMode` configuration option

[thescientist13]

Type of Change

• Other (please clarify below)

Summary

Per #49 / #50 , it seems that there was a much easier solution to the problem of wanting static content in the Light DOM all along; just use innerHTML 😅

Details

I think given how using innerHTML is just standard DOM and per #50 allows each component to opt-in individually, it seems to make this configuration option irrelevant, for now at least. So should

1. Refactor website to use innerHTML
2. Update compiler to drop support for lightMode configuration and remove from build.js
3. Update documentation to remove reference to lightMode
4. Open up a discussion to capture the original intent / use case, just for posterity in case its something worth re-investigating
5. Update README
6. Not sure if this implicates this getInnerHTML issue at all?