feat(bash): Add pnpm

[ExE-Boss]

Since #1731 added yarn to prism-bash.js, this adds highlighting for pnpm, which is the third largest Node package manager.

pnpm’s command‑line syntax is otherwise the same as for npm, and as it’s the third largest, I believe highlighting support for it should be added.

See also:

• https://www.kochan.io/nodejs/why-should-we-use-pnpm.html
• https://medium.com/pnpm/flat-node-modules-is-not-the-only-way-d2e40f7296a3

/cc @zkochan

[RunDevelopment]

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