fix(node): remove browser SDK dependencies

[leoromanovsky]

Motivation

@datadog/flagging-core@2.0.1 and @datadog/openfeature-node-server@2.0.1 both declare @datadog/js-core@0.0.3. That package is published from DataDog/browser-sdk; its only use in these packages is the timestamp type and helper.

The shared evaluator and Node provider must not install browser SDK runtime dependencies.

Changes

• Define the branded epoch-millisecond TimeStamp type and export timeStampNow() from the @datadog/flagging-core root API.
• Reuse the core-owned helper for evaluator, aggregation, and Node provider exposure timestamps.
• Remove @datadog/js-core from the core and Node package manifests and lockfile edges.
• Extend the existing packed Node installation test with a dependency-tree purity check.
• Fail CI if any installed Node dependency is @datadog/js-core, a known browser package, or declares DataDog/browser-sdk as its repository.

Decisions

• Own the timestamp primitive in runtime-agnostic flagging-core instead of importing it from the browser SDK.
• Preserve the existing new Date().getTime() behavior and branded timestamp values behind the shared helper.
• Keep @datadog/openfeature-browser unchanged; it may depend directly on browser SDK packages.
• Enforce purity on the packed, installed dependency tree so transitive browser dependencies are rejected as well as direct ones.
• Preserve the existing timestamp wire values and evaluator-reason behavior.