feat: Support light and dark brand colors

[gadenbuie]

Companion to posit-dev/brand-yml#120.

Summary

Adds light/dark brand.yml color and typography support to bs_theme(brand = ...), using Bootstrap's existing data-bs-theme runtime contract.

• Keeps the current Sass compilation as the default/light theme and appends a dark-mode compilation only when the brand defines mode-dependent values.
• Recompiles Bootstrap component and utility rules under [data-bs-theme="dark"] so Sass-derived styles such as buttons update together with CSS custom properties.
• Preserves scalar brand behavior and leaves Bootstrap defaults intact when a light or dark branch is undefined.
• Preserves brand metadata and HTML dependencies through public theme mutators and the appended dark CSS cache artifact.
• Handles expanded and compressed Sass output with charset or BOM prefixes.
• Requires the development brand.yml version that provides the paired light/dark Sass contract.

Verification

brand <- brand.yml::as_brand_yml(
  "
  color:
    primary: { light: '#0d6efd', dark: '#6ea8fe' }
    background: { light: '#ffffff', dark: '#212529' }
    foreground: { light: '#212529', dark: '#f8f9fa' }
  "
)
theme <- bslib::bs_theme(brand = brand)

bslib::bs_theme_dependencies(theme)
# Generated CSS responds to:
#   document.documentElement.dataset.bsTheme = "light"
#   document.documentElement.dataset.bsTheme = "dark"

• Focused brand theme tests pass with 182 expectations.
• Full devtools::test() suite passes with 1,305 expectations.
• Browser coverage verifies computed button, body, and link styles while switching modes at runtime.
• Air formatting and git diff --check pass.
• R CMD check --no-manual passes with 0 errors, warnings, or notes.