chore(CLI Tool): #3591632 Change default global CSS path in CLI and Workbench config

• Change default globalCssPath to src/global.css
• Normalize path definitions in all canvas.config.json shapes, e.g.: ./src/[...] → src/[...]
• Add backwards compatibility and warning for codebases keeping CSS at src/components/global.css with no globalCssPath in canvas.config.json.

Testing steps

1. Put global CSS in the new default location: src/global.css

2. Run Workbench and a Canvas CLI command, for example:

npx canvas-workbench
npx canvas validate

3. Check that:

   • Both commands work normally.
   • No warning appears.
   • Global styles appear in Workbench previews.

4. Move the file to the old location:

   src/components/global.css

   And remove:

   src/global.css

5. Run Workbench and the Canvas CLI command again.

6. Check that:

   • Both commands still work.
   • A warning appears about the old global CSS location.
   • Global styles still appear in Workbench previews.

7. Add this to canvas.config.json:

     {
       "globalCssPath": "src/components/global.css"
     }

8. Run Workbench and the CLI command again.

9. Check that:

   • Both commands still use the old location.
   • The warning no longer appears.

Closes #3591632 (closed). Authored with an AI coding agent.