Add modal dialogs

[yscik]

Part of 186-gh-Automattic/wpjm-addons

Includes some related CSS fixes that came up along the way.

Changes Proposed in this Pull Request

• Add a new modal dialog class that can open a popup modal in an overlay
• Uses the native <dialog> element
• Add styling and scripts to open and close the dialog
• Move styles and code around for some elements (buttons), to share it between notice, dialog and other future classes
• Fix the buttons and links not picking up theme colors
• Fix alignment issue in hint notices (438-gh-Automattic/wpjm-addons#issuecomment-1892794397), and add a new variation for 436-gh-Automattic/wpjm-addons#discussion_r1453638803

Testing Instructions

• Add the [job_manager_ui_test] shortcode to a page, publish it and open it
• First element should have an 'Open Modal' button, use it to open the modal dialog
• Clicking outside, on the X or the Close button should close the modal
• Double-check that the rest of the notices keep working

Release Notes

• Add support for modal dialogs

New or Updated Hooks and Templates

Screenshot / Video

Desktop:

Mobile:

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Plugin build for ab543ddaa4f796fbe2019c38a2dc35dada63ac1e
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[yscik]

Fixed style loading for non-block themes in ead3c17, and rebased to latest trunk.