Add Canvas Share component and hide the Share block in Drupal Canvas

Problem/Motivation

Drupal Canvas currently exposes two "Share" entries in its component library:
the legacy Webshare block and the Share SDC. Editors get a duplicated, confusing
choice in the component picker. Behaviour, look, and settings differ between the
two paths.

The approved Figma design and the rest of the Vartheme/Varbase Canvas authoring
experience expect a single, themed Share component with predictable settings — not
a block-based control that exists alongside a parallel SDC.

Proposed resolution

• Add the Share component (SDC) to Varbase and
  Vartheme as a first-class Drupal Canvas component.
• Mirror the legacy Share block settings on the SDC 1:1 — Title, Display title,
  Alignment, Orientation, Mobile visibility, Native share button, Placement — so
  editors keep the same authoring controls.
• Hide the legacy Webshare Share block from the Canvas component library
  (the block remains installed for backward compatibility; it is just not listed).
• Have the component auto-fill the share URL and the enabled social platforms
  for the current page, so it works out of the box with no manual configuration.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release varbase-11.0.0-alpha3, varbase_starter-1.0.0-alpha2

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• chore: #3591822 Add Canvas Share component and hide the Share block in Drupal Canvas