Add ssg.running flag

[simonhamp]

Use-case

I have a site that uses the SSG but also runs fully dynamically elsewhere. Having this flag enables me to cut out/replace parts of templates that are 'too' dynamic with something more predictable for the SSG-generated version without having to completely remove chunks of template manually.

The case given in the readme is the most pressing example, but I'm sure there will be others.

Why a dynamic config key?

A static one will be present all of the time, so always true or always false. It could be added to the file, but that seems a bit redundant - no one should ever tamper with it.

Why not an env variable or some other variable?

Mainly down to antlers support - I wanted something I could access easily in my templates without much fuss and without polluting the global namespace with another variable.

The config array is readily accessible for this - and I checked: this still works even after running php artisan config:cache.

[simonhamp]

Following some discussion on Discord, I'm closing this in favour of an alternative approach, that doesn't use a dynamic config key