Modernize the path alias system

Problem/Motivation

As part of the Workflow Initiative, we want to improve the path alias system and make it easier to integrate into modern Drupal workflows, like the new Workspaces or Content Moderation modules.

The current system has various deficiencies:
- it uses a custom storage pattern
- the integration with other entity types is done via a custom (computed) field type, which is an endless source of tricky bugs
- path aliases are not revision-aware nor publishable, which means they can not be integrated into the other core workflows

Proposed resolution

Do all the things!

Remaining tasks

Conversion to revisionable entities:
#3006086: update.php should not process path aliases
#2336597: Convert path aliases to full featured entities
#3084983: Move all the code related to path aliases to a new (required) "path_alias" module
#3009014: Convert path alias migrations to use entity:path_alias destination
#3007832: Convert custom path alias forms to entity forms
#3011807: Convert the path alias admin overview to a list builder
#2233595: Deprecate the custom path alias storage

To be done after the initial conversion:

#3007669: Add publishing status to path aliases
#3008058: Convert the computed path item field type to a regular entity reference field

User interface changes

Various features that are available for all content entity types will also be enabled for path aliases, like Views integration, etc.

API changes

A new core entity type with a backwards compatibility layer in place until Drupal 9.

Data model changes

Path aliases will be converted to revisionable and publishable entities, and the core Path module will be required.