Release Volt 0.20.0

[dannote]

Prepares 0.20.0, which contains #49, #50 and #51.

• @version to 0.20.0.
• The changelog's Unreleased section becomes 0.20.0 - 2026-10-03. Its Added entries are reordered so the reload changes read together, and the reload_dirs entry notes that the watcher now broadcasts document instead of full.
• Install snippets in the README and guides point at ~> 0.20.

A minor bump because of two breaking changes: formatter options move to the :volt key of .formatter.exs, and externals stay as imports in :esm and :cjs output.

Checked locally

• mix test: 755 passed. Browser tests (--include integration): 30 passed. mix lint passes.
• mix hex.build includes the newly vendored priv/ts/node_modules/idiomorph, which the dev client needs.

After merging

Not done here: tag v0.20.0, mix hex.publish, and the GitHub release. Run mix volt.priv.vendor priv/ts before publishing so the package ships the vendored client packages.