Fix build on Windows, synchronize .gitignore and README.md

[lauriharpf]

Cross-platform compatibility

Makes pnpm run build work on both macOS and Windows platforms. While creating the reproduce repo for web-infra-dev/rspack#8170, I noticed that pnpm run build worked on macOS but failed on Windows.

On Windows, the error messages from pnpm run build initially were

Unknown arguments: =1, pnpm, rspack
 ELIFECYCLE  Command failed with exit code 1.
ERROR: "build:rspack" exited with 1.
 ELIFECYCLE  Command failed with exit code 1.

Those errors were fixed by adding cross-env. After that, the build:rsbuild step still failed on Windows with the following error message (fixed with the --env-mode development):

  Rsbuild v1.0.1
  
error   Failed to build.
error   'local' cannot be used as a value for env mode, because ".env.local" represents a temporary local file. Please use another value.

.gitignore and README.md synchronization

Remove rspack-dist and webpack-dist from .gitignore to be in sync with README.md ("./webpack-dist and ./rspack-dist are purposely not added to .gitignore.")

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