feat(scaffolder): add new plugin for extending scaffolder actions

This MR adds a new backstage backend plugin, which will be used for installing custom scaffolder actions for our Backstage Software templates.

The first released action will focus on HCL file merging, which will be useful for bootstrapping terraform on existing terraform projects.

This includes:

Referencing a new package @seatgeek/node-hcl, which is a WASM wrapper around hashicorp/hcl for manipulating HCL files.
Use the package's exposed Merge function.
A new Backstage plugin package under plugins called scaffolder-backend-module-hcl.
The Backstage plugin package will expose actions such as hcl:merge hcl:merge:write hcl:merge:file hcl:merge:file:write.
Unit tests for the plugin.
Updated add-copyright script for including Go source files as part of the check.

seatgeek / backstage-plugins