Add --parallel flag to lerna exec

[evocateur]

Description

With this flag, lerna exec will run the command in all filtered packages
in parallel, completely ignoring concurrency and topological sorting.

# transpile modules in all packages as changes occur
lerna exec -- babel src -d lib -w

# transpile watched modules only in package-foo
lerna exec --scope package-foo -- babel src -d lib -w

Motivation and Context

I found this useful in a hoisted monorepo to transpile package exports that are themselves linked to private webpack entry points. Disabling the progress bar is necessary in all cases.

It is advised to constrain the scope of the command when running with this
flag, as spawning dozens of subprocesses may be harmful to your shell's
equanimity (or maximum file descriptor limit, for example). YMMV

How Has This Been Tested?

local development in a lerna monorepo, and several integration tests.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[lock]

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