feat(notes): search notes with VS Code's search

[dionmunk]

There was no way to search inside notes (#17). VS Code's Find in Files only covers the workspace, and notes usually live outside it.

Changes

• Notes: Search Notes in the command palette opens VS Code's Search view limited to your notes: Workspace Notes and Notes when the workspace has notes, otherwise Notes.
• A search button in each section's title bar searches that section (title bars now read New Note, New Folder, Search, Refresh).
• Search in Folder in a folder's right-click menu searches that folder.

The folders go into "files to include", so everything Search offers works on notes: regex, match case, replace across notes, and results grouped per folder.

What I checked before building it

A probe extension ran workbench.action.findInFiles in VS Code 1.140 with the notes folder as filesToInclude. VS Code searched it when it was outside the workspace, when no folder was open, and when it was a .notes folder listed in the project's .gitignore, so ignore files don't need turning off (doing that adds an "exclude settings and ignore files are disabled" notice). Two folders separated by a comma were searched together.

Testing

• npm test: 29 passing (no new unit tests; the change is command wiring around VS Code's search).

• End to end in VS Code 1.140 on macOS, searching for a word that is in three notes and in a project file:

  Scenario	Files to include	Files found
  Notes: Search Notes, with Workspace Notes	.notes, my-notes	project.md, animals.md, more.md
  Notes title bar search	my-notes	animals.md, more.md
  Workspace Notes title bar search	.notes	project.md
  Search in Folder on sub	my-notes/sub	more.md
  no folder open	my-notes	animals.md, more.md
  no storage location		Search not opened, "You need to select a location to store your notes."

  The project file with the same word was never in the results.

• Not handled: a notes path that contains a comma, braces or brackets would be read as part of a pattern by VS Code's include field. Not tested on Windows or Linux.

Closes #17.