Support detecting args that is specified but with no values

Describe the feature

Generally I have to using customized args parser to write tje following code. I was hope that citty could support the requirement said in the title, since it already use mir under the hood.

import process from 'node:process'
import parseArgs from 'mri'
import { defineCommand, runMain, showUsage } from 'citty'
import { ensureConfig, logger } from './utils.js'
import { bundle } from './commands/bundle.js'
import {
  version,
  description,
} from '../../package.json' assert { type: 'json' }
import { DEFAULT_CONFIG_FILENAME } from './constants.js'

interface ParsedArgs {
  config?: string | true
  c?: string | true
  // `citty` intercept the help option, so we don't need to deal with it
  // help?: boolean
  // h?: boolean
}

const main = defineCommand({
  meta: {
    name: 'rolldown',
    version,
    description,
  },
  args: {
    config: {
      type: 'string',
      alias: 'c',
      description:
        'Use this config file (if argument is used but value is unspecified, defaults to `rolldown.config.js`)',
    },
    help: {
      type: 'boolean',
      alias: 'h',
      description: 'Show this help message',
    },
  },
  async run(_ctx) {
    // FIXME: `citty` doesn't support detecting if an argument is unspecified
    const parsedArgs = parseArgs<ParsedArgs>(process.argv.slice(2))
    let argConfig = parsedArgs.c || parsedArgs.config
    if (argConfig) {
      // If config is specified, we will ignore other arguments and bundle with the specified config
      if (argConfig == true) {
        argConfig = DEFAULT_CONFIG_FILENAME
      }
      await bundle(argConfig)
      return
    }

    showUsage(main)
  },
})

runMain(main)

Additional information

• Would you be willing to help implement this feature?

Start by inspecting citty's argument parsing around defineCommand and runMain, then compare how the bundled mri parser represents an option with no value. Add coverage for distinguishing an explicitly provided option from an omitted one, including the config example, and verify the existing CLI behavior remains intact.