Add support for pnpm

[valerybugakov]

Context

This PR adds support for pnpm workspaces. We're switching to pnpm to simplify our integration with Bazel. See the tracking issue for more context:

• https://github.com/sourcegraph/sourcegraph/issues/44909

Test plan

1. Checkout to vb/pnpm-test-3 in the Sourcegraph monorepo.
2. rm -rf node_modules ./client/*/node_modules && pnpm install
3. DIR=/path/to/scip-typescript; "$DIR/node_modules/.bin/ts-node" "$DIR/src/main.ts" index --pnpm-workspaces

[valerybugakov]

Looks like the CI / test (18.x) job fails in other PRs not related to the package logic too.

[olafurpg]

We can add an integration test by adding a pnpm project under snapshots/input and make sure to enable the new --pnpm-workspaces flag here

scip-typescript/src/main.test.ts

Line 57 in 4834ff7

yarnWorkspaces: Boolean(packageJson.workspaces),

[valerybugakov]

@olafurpg, added 56506ad 👍

[valerybugakov]

And fixed the issue with the CI / test (18.x) job.