[6.x] Support for multiple conditions with the same target field

[godismyjudge95]

This came up in the Statamic Office Hours, and it's something I've personally run into a number of times.

Currently when you want to reveal based on a field being A or B or C. You must create a custom condition because Statamic doesn't support using the same field for multiple conditions. This PR fixes that.

It is completely backwards compatible and simply wraps the right hand side of the condition in an array if there are multiple conditions for the same target field.

This outputs this for multiple conditions:

if:
  age:
    - '> 18'
    - '< 65'

And this for a single condition:

if:
  age: '> 18'

Any previous blueprints with only a single condition will continue to work and if a new condition is added to them it will convert it to the array/list syntax.

Resolves:

• Allow configuring multiple conditions for the same field ideas#1101
• Conditional fields: can't match on two options of a select field #759
• Conditionals on Field Not Persisting Correctly #6008