fix: avoid endless vite processes when Vite is missing

[MatheusRich]

The runner starts Vite through the package manager, for example bun x --bun vite. When the vite package is not installed, Bun and pnpm look for vite on the PATH. There they find the vite executable of this gem, which starts the runner again. Each process waits for its child, so the processes accumulate without end.

This commit adds a marker variable to the environment of the process that the runner starts. When vite build or vite dev sees the marker, it stops with an error. The error tells the user to install the JavaScript packages. vite-plugin-ruby removes the marker when Vite loads, so a process that Vite starts can still run bin/vite.

The check runs before Builder#build. With the build lock of PR #630, a later check would deadlock instead of failing. The marker has no VITE_ prefix, because Vite copies those variables into import.meta.env.

A check for node_modules/.bin/vite is not possible, because Yarn Plug'n'Play has no node_modules. A check of the parent PID does not work either, because pnpm starts the child through two processes.

With an older vite-plugin-ruby, vite build and vite dev fail when a process that Vite started runs them. DEFAULT_PLUGIN_VERSION should move to the next plugin release to prevent this.