Replace the legacy ToolbarButton instances for toggling the navigator and debugger sidebars with declarative Lit devtools-button elements. This updates TabbedEditorContainer to accept Lit templates for toolbar items and simplifies SourcesView's layout state and toolbar rendering logic.

Bug: 407751445
Change-Id: I1400d5b949ca9125ba31672bc4f605498cf45a7c
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8445663
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Danil Somsikov <dsv@chromium.org>