Added pnpm package manager + refactored package managers#1022

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yuyaryshev wants to merge 1 commit intojaredpalmer:masterfrom yuyaryshev:add-pnpm-package-manager

+100-46

Commits on Apr 27, 2021

- Added pnpm package manager. Refactored package managers' code to be all inside one file - getPackageManager.ts. Added checks for lock files to decide which package manager to use.
yuyaryshev
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