process: migrate CI from Kokoro to LUCI

[hyangah]

Move our primary CI to LUCI, the default CI & trybot infra in the Go project.
That will help us align our test workflow with other Go projects, simplify integration with Gerrit & Relui.

• In addition to the go toolchain and git, vscode-go project requires extra tools in the builder image.
  • Node.js (npm)
  • xvfb (xvfb-run) for testing in linux
  • access to the network

Other tools can be built with go install and npm ci+npx on demand.

• We will also need to figure out how to present the vscode-go integration test output.

• Nice to have:
  Windows/Mac support
  Trybot

cc @golang/release @golang/tools-team

[dmitshur]

Adding some relevant notes here.

CIPD as used by LUCI builders already has an existing package providing Node.js, and it's used for testing the js/wasm port: https://chrome-infra-packages.appspot.com/p/infra/3pp/tools/nodejs.

For something like xvfb, it might work better to add it to the image (e.g., see here).

*-longtest builders have internet access by default (note that go test -short isn't expected to need internet, so tests that require internet are expected to skip when testing.Short() is true).