code: Add icon to toolbar for markdown files that opens the preview

Whereas the reStructuredText part of the extension could copy the
patterns for previews established by VSCode's markdown support, doing
so for markdown files would (surprise, surprise!) create conflicts.

So, the toolbar button and command names have been "re-branded" to
make them distinct from the built-in markdown ones. The
reStructuredText part of the extension keeps its existing
keybindings (for now?) and no keybindings have been set for the
markdown side.

Open for suggestions as to what a good default keybinding could be :)

Ref: swyddfa#780

code/changes/780.breaking.md

@@ -0,0 +1 @@
+The `Open Preview to the Side` and `Open Preview` commands have been renamed to `Preview Documentation in Split Window` and `Preview Documentation` respectively.

code/changes/780.enhancement.md

@@ -0,0 +1 @@
+Esbonio now adds an icon to the editor toolbar that opens the preview in markdown files.

code/package.json

@@ -64,14 +64,14 @@
  "commands": [
  {
  "command": "esbonio.preview.open",
- "title": "Open Preview",
- "icon": "$(preview)",
+ "title": "Preview Documentation",
+ "icon": "$(open-preview)",
  "category": "Esbonio"
  },
  {
  "command": "esbonio.preview.openSide",
- "title": "Open Preview to the Side",
- "icon": "$(open-preview)",
+ "title": "Preview Documentation in Split Window",
+ "icon": "$(book)",
  "category": "Esbonio"
  },
  {
@@ -418,7 +418,7 @@
  "command": "esbonio.preview.openSide",
  "alt": "esbonio.preview.open",
  "group": "navigation",
- "when": "resourceLangId == restructuredtext"
+ "when": "resourceLangId == restructuredtext || resourceLangId == markdown"
  }
  ]
  },