feat: add feature plugin tests and validation for feature flags

vite-layers/bin/vite-layers.mjs

@@ -1,11 +1,11 @@
 #!/usr/bin/env node
-// CLI for vite-layers. Loads the TypeScript source via jiti (no build step needed).
+// CLI for vite-layers. In a source checkout it loads the TypeScript source via jiti (no build step
+// needed); a published package ships `dist` (not `src`), so it falls back to the built `dist/index.js`.
+import { existsSync } from 'node:fs'
 import { fileURLToPath } from 'node:url'
 import { dirname, resolve } from 'node:path'
-import { createJiti } from 'jiti'
 
 const here = dirname(fileURLToPath(import.meta.url))
-const jiti = createJiti(import.meta.url)
 
 const [cmd, appArg] = process.argv.slice(2)
 
@@ -14,7 +14,11 @@ if (cmd !== 'prepare') {
   process.exit(cmd ? 1 : 0)
 }
 
+const srcEntry = resolve(here, '../src/tsconfig.ts')
+const { writeTsConfig } = existsSync(srcEntry)
+  ? await (await import('jiti')).createJiti(import.meta.url).import(srcEntry)
+  : await import(resolve(here, '../dist/index.js'))
+
 const appDir = resolve(process.cwd(), appArg ?? '.')
-const { writeTsConfig } = await jiti.import(resolve(here, '../src/tsconfig.ts'))
 const file = await writeTsConfig(appDir)
 console.log(`vite-layers: wrote ${file}`)