rustdoc: Restore --default-theme, etc, by restoring varname escaping

In #86157

    cd0f93193c
    Use Tera templates for rustdoc.

dropped the following transformation from the keys of the default
settings element's `data-` attribute names:

    .map(|(k, v)| format!(r#" data-{}="{}""#, k.replace('-', "_"), Escape(v)))

The `Escape` part is indeed no longer needed, because Tera does that
for us.  But the massaging of `-` to `_` is needed, for the (bizarre)
reasons explained in the new comments.

I have tested that the default theme function works again for me.  I
have also verified that passing

    --default-theme="zork&"

escapes the value in the HTML.

Closes #87263.

CC: Jacob Hoffman-Andrews <github@hoffman-andrews.com>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

src/librustdoc/html/static/js/storage.js

@@ -22,6 +22,8 @@ function getSettingValue(settingName) {
         return current;
     }
     if (settingsDataset !== null) {
+        // See the comment for `default_settings.into_iter()` etc. in
+        // `Options::from_matches` in `librustdoc/config.rs`.
         var def = settingsDataset[settingName.replace(/-/g,'_')];
         if (def !== undefined) {
             return def;