feat(notes): show recently opened notes first in List Notes

[dionmunk]

A note opened often, like todo.md, sat wherever its name put it in Notes: List Notes (Alt+L) (#18). Like VS Code's Quick Open, the notes you opened most recently now come first.

Changes

• Recently opened first. With nothing typed, the list starts with the notes opened most recently, newest first, under a "recently opened" heading, then every other note in name order under "other notes". Without any history the list is unchanged.
• Opened from anywhere. A note counts as opened when it becomes the active editor, whether that's from Notes, List Notes, the Explorer or Quick Open, including when it opens in a preview or a viewer.
• Kept on this machine. The last 50 notes are stored in the extension's global state, not in settings or the notes folder, and are not synced. The history follows notes and folders that are moved, and deleted notes are left out.
• Notes: Clear Recently Opened forgets the history.

While typing

VS Code ranks matches itself while you type (keeping our order would need sortByLabel, which is still a proposed API), so a better match comes before a recent one. With the headings in place, "recently opened" then labelled notes that weren't recent, so the headings are only shown for the full list. Once you type, it's VS Code's ranked matches, as in any quick pick.

Testing

• npm test: 35 passing, 5 new tests for the history (newest first without duplicates, the limit, a moved note, a renamed folder versus a folder with a similar name, clearing). With the folder check loosened to a plain prefix, the renamed-folder test fails.

• End to end in VS Code 1.140 on macOS:

  Scenario	List Notes
  nothing opened yet	project.md, alpha.md, b-todo.md, todo.md, zz.md (no headings)
  opened zz.md, then todo.md	recently opened: todo.md, zz.md; other notes: the rest
  type to	todo.md, b-todo.md, Create entries, no headings
  opened project.md from the Explorer	project.md first under recently opened
  restarted VS Code with the same profile	same order, the history was kept
  dragged todo.md into archive	still in recently opened, at its new place
  Notes: Clear Recently Opened	"Cleared the recently opened notes.", back to name order without headings

• The List Notes scenarios from Combine search for notes and create new notes #34 (create from the list, typed extensions, both sections, Escape) gave the same results.

Closes #18.