Fix dev-server and test-runner bugs; move formatter options; Rollup-style externals - #49

A second watcher with different options, such as a Mix.Tasks.Volt.Dev
entry in the endpoint's :watchers next to plug Volt.DevServer, made
every request answer a bare 503. The response and an error log now name
the conflict and how to resolve it.

Phoenix initializes plugs on every request in development, so the dev
server logged "Pre-bundled N vendor package(s)" per request even when
the cache was fresh. The log moves to where bundling happens.

Chunks already read their external globals inside the IIFE. A second
copy was prepended at the top level of each script, where two chunks
importing the same name would clash.

During application shutdown a watcher can terminate after
Volt.Tailwind.Registry has stopped, and releasing its context raised
"unknown registry". There is nothing left to release then.

Source lines were matched to collected tests by position with Enum.zip,
which truncates to the shorter list. The line scan only recognized plain
test() and it() calls, so a five-case table followed by two tests was
collected as two tests.

Each case now maps to the line of its table, and describe.each repeats
its inner tests per case. When the source shows a different number of
tests than the runtime collected, such as tests registered in a loop,
all tests are kept without a line.

The client types only declared import.meta.hot, so TypeScript projects
had to declare ImportMeta.env themselves. The installer adds env.d.ts to
configurations that list declaration files explicitly.

They were not in the dev server's compilable extensions, so the private
extension list in Volt.JS.CommonJS could never match .cjs. Both are
CommonJS by extension and go through the same conversion as CommonJS
.js files; only .js is inspected. The list comes from
Volt.JS.Extensions.

The watcher supervisor started before the Tailwind registry and worker
supervisor, so on shutdown the Tailwind processes stopped first and
every watcher with Tailwind enabled crashed in terminate/2 with
"unknown registry: Volt.Tailwind.Registry".

Start the Tailwind processes first. This replaces the Process.whereis
guard in Volt.Tailwind.Supervisor.release/1, which hid the ordering
problem.

Volt.Test.Lines goes back to OXC.postwalk/3 instead of a hand-rolled
traversal. Post-order reaches the tests in a describe.each body before
the describe.each call, which folds them into one repeated group.

Line numbers were found by counting newlines from the start of the
source for every test. On a 110 KB file with 1,200 tests that took
about 140 ms of 214 ms; a single pass over the newline offsets brings
the whole function to about 81 ms, most of it OXC.parse.

mix lint failed on Elixir 1.20 / OTP 29, the job that runs it:

- Credo: two single-item list appends.
- Architecture policy: Volt.JS.CommonJS had no layer, and Volt.Config
  (model) called Volt.JS.Format (logic), closing a layer cycle. The
  check for formatter options left under config :volt, :format moves to
  Volt.Config, which Volt.JS.Format calls.
- Smells: a case returning booleans in the dev server, and a member
  expression shape repeated across the callee matchers in
  Volt.Test.Lines, now one function.

Volt's .formatter.exs held the formatter options but did not list
Volt.Formatter, so mix format never exercised the plugin on Volt's own
sources. Add the plugin and the priv/ts sources that mix volt.js.format
already covers.

mix format loads a project's own formatter plugin from the existing
build and compiles only when the module cannot be loaded. CI restores a
cached _build, so it formatted Volt's TypeScript with the previous
Volt.Formatter, which did not read options from .formatter.exs, and
reported every file as unformatted.