Refactor merge scope

[guillemcordoba]

Depends on #155

Refactors the merge_scope helper into two separate ones. Here is the new documentation for them.

Without this, the use of merge_scope turns out to be quite constrained in some use cases, particularly when the template needs to merge two scopes at the same level. As an example:

export class A {
}
export class B {
}

With the old merge_scopes, it was impossible to add a new function to the end of both A and B, you had to choose between one of them. Now with the new helpers the templates can do it like this:

{{#merge previous_file_content}}
  {{#match_scope "export class A"}}
    {{previous_scope_content}}
    
    newFunctionA() {}
  {{/match_scope}}
  {{#match_scope "export class B"}}
    {{previous_scope_content}}
    
    newFunctionB() {}
  {{/match_scope}}
{{/merge}}

This has no impact on the built-in templates as they don't use this helper, but this custom template does. I'm the maintainer of that template and I need this change for it to work well.