Add eventcart shell

[eileenmcnaughton]

Overview

This adds the shell of the event cart extension - at this stage it's purely civix generated and is a component portion of
#17508

There is a long-standing technical desire to separate core out into a more modular and less interdependent structure. This includes such things are using the same methods available to extensions for parts of core to interact with each other and potentially, eventually, allowing portions of it to be disabled by some sites or even moved out of the core repo to be managed by other maintainers at a different development pace (in some cases this would be part of a obsolescence strategy - e.g the core implementation of PaymentExpress (NZ payment provider) is superceded in the Omnipay extension so the eventual goal of a strategy to extensionize that processor would be for it to no longer be in core or maintained

Before

No steps yet merged towards the goal of separating out event cart

After

A shell extension is not in the ext directory - the various code needs to be moved into this extension and we need routines to enable it. However, the challenges of improving civix to the point where a new extension would pass code style were not trivial and we are treating that as step one.

Technical Details

Generated with the command civix generate:module eventcart --author="CiviCRM" --email="info@civicrm.org"

Comments

Adding the generated code first makes later changes very transparent / reviewable

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

@seamuslee001 @totten looks like this will pass now - yay - this is pure civix !

Next step is to move BAO / tpl files & enable on install / upgrade. At that point we will have the bulk of the straight move done and then @mattwire can PR in all the improvements without ongoing staleness / huge unreviewable PRs.