justfile: Add recipes to preview without any user config loaded #128
Description
The current just *preview recipes try to use the syntaxes from this repo with all other (non-justfile-related) user configuration, so that things like color schemes are preserved. Unfortunately, some user configurations somehow override parts of this repo (e.g. user setups that include vim-polyglot), creating only a "partial preview" that is just confusing. It would be useful if our repo justfile provided a sure way to bypass these type of configs.
The behavior of the existing preview recipes is intentional and should not be changed, but we could add "clean preview" recipes that actively avoid loading any user config. Recipes like that could also be useful troubleshooting tool for some types of support questions.