Text input + autosuggest for New Workspace modal UI

[jldec]

This is the first step to improve the UX for startig new workspaces in the dashboard. Specifically this removes the 2 tabs in the existing modal UI, and replaces them with a single input field with auto-suggest. The modal behavior (grey background) can remain the same. For additional context, see #7554

Proposed UI changes

The new design should have only the following elements

• A single text input with the ability to paste a repo context URL or type some text
• An autosuggest feature with selectable results for a repo context which are based on what is in the text input
• Short instructions for how to use:
  Paste a repository context URL or start typing to see suggestions. Learn more

Repo context requirements

The input should accept the same repo context URLs as currently supported for prefixed URL workspace starts.

Autosuggest requirements

When the input text matches, autosuggested results should include at least the following:

• recent workspaces
• examples (like today)
• personal account repositories from your git provider(s)

Nice to have

• A dashboard keybinding (exact characters TBD) similar to a command pallette reachable from anywhere in the Dashboard.

[jldec]

Visual design guidelines for this first implementation from the sync today:

• Keep the current modal dimensions and scrolling behavior and list size limit unchanged
• Render the auto-suggested results as simple git repo context URLs including the http://git-provider/... prefix
• No need for a submit button - click on any result to submit.

Suggestions for future improvements

• Render results with URL broken down into components: git-provider, user-org/repo-name, issue / PR / commit context
• Provide additional hints in the results for different types of repos e.g examples, recently used, personal, org
• Explore alternatives to size-constrained modal UI, including non-modal page with non-nested scrolling

[jldec]

Additional design guidelines from followup sync (internal):

results list design - for this issue

• results are always repos
• single-line result with full URL can be hard to read
  Showing 2 parts - repo (above) + host/org (below) - would much more readable
• avoiding a scrollbar is nice, but it's better to show more results with a scrollbar, than fewer without
• showing only 3 results say with no scrollbar, and also showing that there are 12 total in a message, feels awkward
• keyboard usage is high priority for a feature like this
  • preselect first item
  • type Enter to select and go
  • use up/down arrow key to navigate list

results list design - future requirements

• show issue/PR/commit context in results
  • using colors for context types would be nice (issue, PR etc)
• repo validation - this can be left to workspace open for now
• selecting from existing workspaces - strong requirement 🤔
  → 2-levels of selection required: first select repo, then select from recent workspaces