Edit/Update:
GitHub just announced a VS Code extension which you can use.
Today, we’re excited to announce the release of the public beta of the official GitHub Actions VS Code extension. The extension provides support for authoring and editing workflows and helps you manage workflow runs without leaving your IDE.
In addition to Azeem's answer, you can install YAML
extension for VSCode.
For Jetbrains' tools refer to this comment:
On the bottom right, click on "No JSON Schema"
Then choose "GitHub workflow"
Basically, you want that your IDE or text editor to load Github Actions' JSON schema.