feat: capture watch mode when called via the Rollup API

Summary

Capture this.meta.watchMode when it is available

process.env.ROLLUP_WATCH is only set when using the CLI
- when using watch mode through the Rollup API, this is not set
- but since Rollup 2.14.0, this.meta.watchMode is available
- still check the env variable for backward-compat on older Rollup versions
notably, this is needed to test watch mode, since the tests are done via the Rollup API