Bump version number to 5.5.0-rc.1

[talldan]

Changelog

Features

• Add a new Group block.
• Add vertical alignment support to the Media & Text block.
• Add the image fill option to the Media & Text block.

Enhancements

• Improvements to the Image Block flows.
• Automatically add mailto: to email addresses when linking.
• Add visual handles for side resizers for various blocks.
• Improve the performance of the annotations by avoiding excessive memoization.
• Announce the color accessibility issues to screen readers.
• Add enum block attributes validation to browser block parser.
• Use a consistent grey background in the Shortcode block.
• Improve accessibility of video block select poster image.
• Respect prefers-reduced-motion for fixed backgrounds in Cover block.
• Prevent ArrowLeft key press in multi-line selection from prematurely triggering multi-selection.

Bug Fixes

• Avoid keeping the RichText value in cache indefinitely.
• Fix the post title input borders in the code editor.
• Fix the block restrictions to insert, replace or move blocks.
• Select gallery images on focus.
• Fix removing gallery images on delete/backspace key press.
• Fix small visual regression in the block autocompete popover.
• Fix the data module resolver resolution status.
• Avoid saving metaboxes when previewing changes.
• Fix selecting the separator block.
• Fix displaying the color palette tooltips on hover.
• Fix Firefox/NVDA bug not announcing the toggle settings button.
• Fix arrow navigation in paragraph blocks with backgrounds.
• Fix the columns block click to select.
• Fix post dirtiness after fetching reusable blocks.
• Fix the hover and focus styles for buttons with the isBusy prop.
• Remove the box shadow from the side inserter button.
• Changing the region navigation shortcuts to avoid conflicts.
• Fix space insertion in the Button block in Firefox.
• Prevent the link popover from animating constantly as we type. 
• Fix the warning triggered by clearing the height of the spacer block.
• Fix undo behavior after fresh post loading.

Documentation

• Add design documentation to the Modal component.
• Clarify the CSS naming coding guidelines.
• Add InspectorControls usage example.
• Tweaks and typos: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13.

Various

• Bootstrap the design of the new widgets screen (non functional yet).
• Reorganization of the block-library code base:
  • Use a babel plugin to load block.json files.
  • Introduce block.json metadata for all client side blocks.
  • Move the edit functions and the icons to separate files.
  • Move the transforms and save functions to separate files.
• Add forwardRef support to the PlainText component.
• Support Button Block Appender in the InnerBlocks component.
• Unset the focal point attributes from the Cover block if not needed.
• Consistently return promises from the data module action calls.
• Remove obsolete CSS currentColor usage.
• Improve the e2e test CLI arguments and docs.
• Improve the e2e test login stability on MacOS.
• Remove is-plain-obj package dependency.
• Remove invalid urls and ids from media test fixtures.
• Remove the deprecated Gutenberg plugin functions slated for 5.4 and 5.5.
• remove or rename undocumented RichText package functions and constants.
• Adjust paragraph block spacing to use standardised variables.
• Allow spaces in file paths for package build process. 
• Update pre-commit to check modified files only.
• Improve the performance of the webpack build configuration.
• Update dependencies with known vulnerabilities.
• Fix typo in variable names.

Mobile

• Accessibility improvements to the Button component.
• Enhance the unsupported block type. 
• Fix image upload progress not being displayed consistently.
• Support copy/pasting images.
• Add the list block.
• Visual refinements for the native nextpage block.
• Fix importing the column block.
• Remove DOM logic from the list block toolbar.
• Put the caret at the end of the text field after merging blocks.
• Fix caret position after inline paste.