Add support for grabbing version from the engines.pnpm field

[Stanzilla]

Currently the action is able to grab the version from the packageManager field which causes a few side effects.

If a user wants the repo to use a specified version of pnpm but not down to the patch level version, it was recommended to set the engines.pnpm field instead since packageManager does not support ranges.

It would be nice if the action supported reading this field as well.

Related:

• Throw error when multiple versions specified #122
• Confusing error messages when user doesn't specify version #54

[karlhorky]

Is this a duplicate of #84 ?

[Stanzilla]

Is this a duplicate of #84 ?

Oh yes, my bad, sorry!