fix: Use posix paths in import statements in Sass strings

[gadenbuie]

Fixes #1681

We are currently relying on os.path.join() and os.path.realpath() which is usually the right choice, but because we're putting the path into another string, we need better escaping for path separators (or better path separators).

The new approach is to use Path(path).as_posix() to convert c:\mydir\...\_01_functions.scss to c:/mydir/.../_01_functions.scss, which then drops into the Sass strings easily.

The code below was run on Windows 11, where the problem is reproducible and highlights the core issue.

from shiny.ui._theme import path_pkg_preset
import sass
import os
from pathlib import Path

path_functions = path_pkg_preset("shiny", "_01_functions.scss")
path_functions
#> 'C:\\Users\\garrick\\Documents\\external\\shiny_css_compilation_demo\\.venv\\Lib\\site-packages\\shiny\\www\\shared\\sass\\preset\\shiny\\_01_functions.scss'
f'@import "{path_functions}"'
#> '@import "C:\\Users\\garrick\\Documents\\external\\shiny_css_compilation_demo\\.venv\\Lib\\site-packages\\shiny\\www\\shared\\sass\\preset\\shiny\\_01_functions.scss"'
f'@import "{Path(path_functions).as_posix()}"'
#> '@import "C:/Users/garrick/Documents/external/shiny_css_compilation_demo/.venv/Lib/site-packages/shiny/www/shared/sass/preset/shiny/_01_functions.scss"'

[cpsievert]

Should we be doing the same for include_paths? Or is the double-quoting not an issue there?

[gadenbuie]

Should we be doing the same for include_paths? Or is the double-quoting not an issue there?

No, those paths are passed to sass.compile() and are use for import resolution. As in the paths are handled entirely in Python. The weirdness happens when we take the path and put it in the string literal containing the Sass code.