fix: ensure string is passed to process.emitWarning

[mdonnalley]

Errors emitted from ts-node are objects with custom inspections instead of errors or strings, which causes this error when emitting the warning:

TypeError: The "warning" argument must be of type string or an instance of Error. Received src/commands/task/get.tsx(2,9): error TS6133: 'Box' is declared but its value is never read.

    at process.emitWarning (node:internal/process/warning:176:11)
    at Plugin.warn (/Users/mdonnalley/repos/oclif/core/lib/config/plugin.js:372:17)
    at /Users/mdonnalley/repos/oclif/core/lib/config/plugin.js:276:30
    at async Promise.all (index 19)
    at async Plugin._manifest (/Users/mdonnalley/repos/oclif/core/lib/config/plugin.js:257:24)
    at async Plugin.load (/Users/mdonnalley/repos/oclif/core/lib/config/plugin.js:208:25)
    at async PluginLoader.loadRoot (/Users/mdonnalley/repos/oclif/core/lib/config/plugin-loader.js:67:13)
    at async Config.load (/Users/mdonnalley/repos/oclif/core/lib/config/config.js:266:27)
    at async Function.load (/Users/mdonnalley/repos/oclif/core/lib/config/config.js:156:9)
    at async run (/Users/mdonnalley/repos/oclif/core/lib/main.js:59:20)

Calling util.inspect on the object gives a string that we can pass in, so the warning now looks like:

(node:92535) Warning: src/commands/task/get.tsx(2,9): error TS6133: 'Box' is declared but its value is never read.
module: @oclif/core@3.19.6
task: findCommand (task:get)
plugin: multiple-repo-manager
root: /Users/mdonnalley/repos/mdonnalley/multiple-repo-manager
See more details with DEBUG=*
Error: command task:get:done not found
    at Config.runCommand (/Users/mdonnalley/repos/oclif/core/lib/config/config.js:402:19)
    at async run (/Users/mdonnalley/repos/oclif/core/lib/main.js:92:16)
    at async main (file:///Users/mdonnalley/repos/mdonnalley/multiple-repo-manager/bin/dev.js:5:3)
    at async file:///Users/mdonnalley/repos/mdonnalley/multiple-repo-manager/bin/dev.js:8:1