Support root config

[huwshimi]

Done

• Add rootPath to support projects that are not at the root of the workspace (fixes: Support vitest.rootPath option #151, fixes: Nested package structure #49).
• Support the Vitest root config for tests that are not at the root of a project.

QA

• Run this extension with the Nested Root Sample project.
• It should find and run the add.test.ts tests.

[ffMathy]

Closing in favor of using vitest.workspace.ts at the root-level.

See more here: https://vitest.dev/guide/workspace

Let me know if you disagree with this decision.