shiny- and shiny-theme- prefixes, add bootstrap page adapter

[gadenbuie]

WIP, mostly for discussion and tinkering. This PR does a few things:

1. First, I added the shiny- prefix to props that are officially part of our design system, e.g. --shiny-text-1 instead of --text-1.

2. I added a new layer of variables that are intended to be "public" facing in the sense that they can be modified at the page level or by page-level CSS. These are prefixed with shiny-theme-, e.g. --shiny-theme-text-1. The explicit goal of these properties is to serve page-level adapters in situations where we want the shiny design system to inherit from the page.

3. The --shiny-* tokens in this system are considered the component-facing properties; these should be used by the web components or by adapters for web components. I updated the shoelace adapter to use these tokens.

The motivation for this is two-fold:

1. First, it increases clarity around the source of our design tokens. Without the shiny- prefix, it can be hard to tell which properties come from open props and which properties are aliases we've created ourselves.

2. CSS variables cannot refer to themselves, so for page-level adapters to set a CSS variable, the fallback value in that adapter cannot refer to the variable being set. Introducing --shiny-theme-* allows page-level adapters to be included in more situations and reduces the risk that an unset variable will break a design token variable.

[wch]

I don’t have strong feelings either way about having a prefix, but I think --shiny- might actually not be quite right, since many of these components will be usable outside of Shiny.