Mailables - Better Placeholder Generator

[skrolikowski]

Describe the issue

Only Eloquent Models are converted into useable placeholders.

Expected behavior

We need to include functionality to generate the following placeholders:

• Single variables (e.g. public $foo = 'bar') => [Single Button]
• Associative arrays (e.g. `public $foo = ['a' => 1, 'b' => 2]) => [Drop-down with options]

Screenshots

Version and browser

Version: v6.0.0 Beta 1
Browser and version: Chrome Latest

Bug report checklist

• I have filled out as much information as I can
• I have detailed how the issue can be reproduced
• I have searched for existing issues and to the best of my knowledge this is not a duplicate