Moving npm-is into pwa-studio.

[revanth0212]

Description

npm-is a utility package that validates if a given script is executed using the given package manager. For instance, pwa-studio can only be installed using yarn and if you try to install using npm it has to fail. @zetlen created this wonderful utility package a while ago which we have been using in the preinstall script to check validate if yarn is being used to install the package. Unfortunately in npm 7 update a critical piece of code that the package relies on has changed. Specifically the value of process.env.npm_execpath when it is invoked using npx. The script extensively relies on the value of process.env.npm_execpath to make the decision about what package manager is used to initiating the script.

Previously (npm 6 or less) if you initiated a script using yarn which internally uses npx the value of process.env.npm_execpath was PATH_TO_THE_YARN_EXECUTABLE but after the update, the value of process.env.npm_execpath has changed to PATH_TO_THE_NPX_EXECUTABLE instead. Due to this the validation has failed and pwa-studio installation fails with the following error:

This PR addresses this issue by removing all references of npm-is and adding a note in the documentation to avoid using npm.

Related Issue

Closes PWA-1660

Acceptance

Should be able to validate if a script is initiated using a given package manager.

Verification Stakeholders

@zetlen
@dpatil-magento

Verification Steps

1. Checkout the branch.
2. Run yarn install and it should work fine.
3. Run npm install and it should fail.

Checklist

• I have added tests to cover my changes, if necessary.
• I have added translations for new strings, if necessary.
• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Messages
📖	Associated JIRA tickets: PWA-1660.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
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[sirugh]

@revanth0212 what's up with this PR?