Theme extensibility

[daveaglick]

Been thinking about how to let theme change settings or even include their own modules or extensions and finally have a scheme I'm happy with.

• The concept of "theme folders" will be introduced.
• The extra "theme" input folder will be removed and instead added as the default "theme folder", but the user can add more to layer themes or theme components.
• Like input folders, theme folders are ordered.
• Any appsettings.json or statiq.json files in the theme folder will be processed.
• Here's the good part: any .csproj files in the theme folder will be compiled (probably via Buildalyzer) and their output included in the generation. That means a theme can define settings in code or just use the .csproj file to include other NuGet packages (which will also get referenced via transitive dependency loading).
  • Need to consider ordering here - we probably want to configure theme through the bootstrapper (I.e., .AddThemeFolder("foo")) but we also need to compile and reference project output, configure input folders, etc. as a result.
• The layouts and other parts of the theme itself should go in a nested "input" folder under the theme folder. This also lets the theme include things like a readme file in the root of it's repository and you can just clone the whole thing right into the theme folder.
• "[theme folder]/input" input paths will be automatically added for all theme folders.

[daveaglick]

Might need to back off the compilation idea :(. There's an ordering problem. If theme paths can be defined via settings and/or the bootstrapper, then by the time we've figured out the theme paths it's too late to load new assemblies and manipulate the bootstrapper from them.

I think the best we can do, at least for now, is allow a variety of appsettings.json etc. files in the theme root. Those will still need to be loaded late, but we can manually merge them with the actual settings. And with computed values now, they can do almost anything a compiled project could do (could even look at letting settings files add pipelines, etc.)

[daveaglick]

Ended up hacking around the ordering limitations described above. Essentially, I moved the file system to be independent of the engine and then split the theme initialization into stages. Themes can now have project files that get compiled and included during execution.