Allow single selection variant for the tree view

Author: HDinger

.changeset/full-states-beg.md

@@ -0,0 +1,5 @@
+---
+'@primer/view-components': minor
+---
+
+Support single selection variant for TreeView

.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/tree_view/multi_select/aria-snapshot--after-interaction.yml

@@ -0,0 +1,3 @@
+- tree:
+  - treeitem "src" [checked] [level=1] [selected]
+  - treeitem "action_menu.rb" [level=1]

.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/tree_view/multi_select/aria-snapshot.yml

@@ -0,0 +1,3 @@
+- tree:
+  - treeitem "src" [level=1]
+  - treeitem "action_menu.rb" [level=1]