fix: use node-fetch only as fallback

[tido64]

Description

Use node-fetch only as fallback when current Node version doesn't implement Fetch API

Test plan

Repeat the following steps in both Node 16 and Node 18:

cd packages/test-app
yarn build --dependencies
yarn start

Once the dev server is running, press j. A message should indicate that we're trying to open the debugger, but nothing will show up. Just verify that it doesn't crash.

In a separate terminal, try to start another dev server:

% yarn start
error Another process is using port 8081. Please terminate this process and try again, or try another port with `--port`.

This one should fail with the error message above.

On Node 16, this is an error message we're trying to avoid:

node:internal/readline/emitKeypressEvents:71
            throw err;
            ^

ReferenceError: fetch is not defined
    at ReadStream.<anonymous> (/Users/tido/Source/rnx-kit/packages/cli/lib/serve/keyboard.js:36:21)
    at ReadStream.emit (node:events:513:28)
    at emitKeys (node:internal/readline/utils:357:14)
    at emitKeys.next (<anonymous>)
    at ReadStream.onData (node:internal/readline/emitKeypressEvents:61:36)
    at ReadStream.emit (node:events:513:28)
    at addChunk (node:internal/streams/readable:315:12)
    at readableAddChunk (node:internal/streams/readable:289:9)
    at ReadStream.Readable.push (node:internal/streams/readable:228:10)
    at TTY.onStreamRead (node:internal/stream_base_commons:190:23)

[kelset]

works as expected, a pity we need to still support Node 16 in 2023 when LTS is 18 🙈