declare engine: node >= 18

[aeschli]

test-cli requires node 18 to run:

When run with node 16:

ReferenceError: structuredClone is not defined
    at DesktopPlatform.prepare (file:///tmp/9511677ff29752e82883f6d8c2c28774ed849526/testCom/node_modules/@vscode/test-cli/out/cli/platform/desktop.mjs:36:22)
    at file:///tmp/9511677ff29752e82883f6d8c2c28774ed849526/testCom/node_modules/@vscode/test-cli/out/bin.mjs:52:38
    at Array.map (<anonymous>)
    at prepareConfigs (file:///tmp/9511677ff29752e82883f6d8c2c28774ed849526/testCom/node_modules/@vscode/test-cli/out/bin.mjs:50:48)
    at runConfigs (file:///tmp/9511677ff29752e82883f6d8c2c28774ed849526/testCom/node_modules/@vscode/test-cli/out/bin.mjs:127:28)
    at main (file:///tmp/9511677ff29752e82883f6d8c2c28774ed849526/testCom/node_modules/@vscode/test-cli/out/bin.mjs:33:26)

I suggest adding an engine entry to test-cli to avoid any confusion with users.

  "engines" : { 
    "node" : ">=18.0.0"
  }