Change field UI so that adding a field is a separate task

Problem/Motivation

• The current field creation workflow in the field UI is somewhat overwhelming to users.
• Users have to step through a series of three forms to add a field.
• Additionally, there are some accessibility concerns with having the "Add new field" and "Reuse existing field" options in the field listing table.
• In #1847590: Fix UX of entity reference field type selection and #1847596: Deprecate Taxonomy term reference field in favor of Entity-reference, @yched (the field maintainer) has suggested moving the "Add field" operation onto its own form so that there is more space to provide a good interaction for creating a new field.

Proposed resolution

• Remove the "Add new field" and "Reuse existing field" rows from the manage fields table.
• Add a local action button for "Add field".
• "Add field" provides a form where the user:
  1. Enters the field label, which automatically generates a machine name as per usual.
  2. Selects either "Create new field" or "Reuse existing field", which reveal options with states...
     • For "Reuse existing field," the option reveals a selection box with the existing fields.
     • For "Add new field," the option reveals the field type selection box as well as the field settings (like maximum # values, entity reference entity type, etc.)
     • For both, the widget selection element is shown.
• For now, submitting this form takes the user to the existing field instance settings form.