meta: add GitHub issue templates — bug report + new node request

## Goal
Structured issue templates reduce noise and make it easy for the community to request new nodes or report bugs in a useful format.

## Templates to create (`.github/ISSUE_TEMPLATE/`)

### `bug_report.md`
- [ ] Describe the bug
- [ ] Steps to reproduce
- [ ] Expected vs actual behavior
- [ ] Node type involved (if applicable)
- [ ] Flow JSON snippet (if relevant)
- [ ] Environment (OS, Node version, Next.js version)

### `new_node_request.md`
- [ ] Node name + category (Trigger / Action / Logic / AI / Data / Messaging)
- [ ] What it does (one sentence)
- [ ] n8n equivalent (link to n8n docs if exists)
- [ ] Key fields the user would configure
- [ ] Any secret/API key fields?
- [ ] Why this node is valuable for Next.js developers

## Notes
- The "new node request" template signals to the community that contributions are welcome and structured
- Doubles as a backlog of community-validated node ideas