Fixes #15. check if helper is defined on controllers

[kazzix14]

Hi. I had the same problem and could not run richer_text:install. I added a small fix to check if helper method is defined or not. This fixes #15.

without this fix.

% rails richer_text:install
bin/rails aborted!
NoMethodError: undefined method 'helper' for class ActionController::API (NoMethodError)

        helper RicherText::TagHelper
        ^^^^^^
Did you mean?  _helpers
...backtraces...

with this fix.

% rails richer_text:install
       rails  railties:install:migrations FROM=active_storage,richer_text
Copied migration 20250325074902_create_richer_text_rich_texts.richer_text.rb from richer_text
Copied migration 20250325074903_create_richer_text_json_texts.richer_text.rb from richer_text
Copied migration 20250325074904_create_richer_text_o_embeds.richer_text.rb from richer_text
      create  app/views/richer_text/contents/_content.html.erb
      create  app/assets/stylesheets/richer-text.css
Adding dependencies to package.json
         run  npm install highlight.js @afomera/richer-text from "."

added 97 packages, and audited 314 packages in 12s

81 packages are looking for funding
  run `npm fund` for details

2 moderate severity vulnerabilities

To address all issues, run:
  npm audit fix

Run `npm audit` for details.
Adding import to application.js
The file /workdir/app/javascript/application.js does not appear to exist