Add missing compilers property to MultiCompiler (#22521)
Add missing compilers property to MultiCompiler (#22521)

Based on the Webpack source, the `MultiCompiler` class has a [`compilers` property](https://github.com/webpack/webpack/blob/69e0844028950e1316fb4ed9f4ac4593ecc205b3/lib/MultiCompiler.js#L21).
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