Make a Go/Hugo module, ensure theme pulls modules

[genebean]

This does a couple of things:

• adds go.mod to root and sets the module line for this theme. This allows the theme to be installed as a module per https://gohugo.io/hugo-modules/use-modules/#use-a-module-for-a-theme instead of being forced to use git submodules or, worse, just copy the files.
• moves default go modules that the theme should import into the theme's config.toml so that any user of the theme will get the needed go modules via running hugo.
• makes the example site pull the main theme as a module
• adds Dependabot so updates are suggested as pull requests automatically and adds go.sum back to make this work right
• makes hugo version consistent across module config and other files

Testing

One way to test this is to cd into the example site, add the line below to go.mod, and run hugo serve -w

replace github.com/statichunt/iwriter-hugo => ../

This method is per https://gohugo.io/hugo-modules/use-modules/#make-and-test-changes-in-a-module

[tfsomrat]

Sorry, We couldn't accept this PR. we have a pattern for our themes. and we can't break that. hope you understand. have a good day.