feat: add snippet quick-pick placeholders

Description

Adds placeholder text for user interactions with snippets, so the user knows what they are selecting.

@tvanderhelm will be interested in the UX ramifications of this work. I don't know what other flows lack user-visible text.

Related Issues

• Related to Snippets: add a loading indicator and cancellation (#1626) but does not completely fix it
• Related to chore: Fix capitalization on placeholder (!2049 - merged) where I asked if more placeholders could be added
• Related to Plan for migrating VS Code user documentation (#1530) which tracks the overall docs migration

How has this been tested?

Screenshots (if appropriate)

Create Snippet

step	before	after
1
2	(tested in workspace with only one project)
3
4

Create Snippet Patch

step	before	after
1
2	(tested in workspace with only one project)
3
4

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes