Merge branch 'npm-support-patch' into 'main'

npm = yarn

watch_script = watch

build_script = build

install_args = --force

```

* The section name `[parcel]` is the name of the folder where the Parcel project is located.

* `npm` is the package manager command used to build the project. This example will use [Yarn](https://yarnpkg.com).

* `watch_script` is the npm script used in `lektor server -f npm`,

* `build_script` is the npm script used in `lektor build -f npm`.

* `build_script` is the npm script used in `lektor build -f npm`,

This plugin supports more than one such entry.

#### install_args

```

[parcel]

npm = yarn

install_args = --force

watch_script = watch

build_script = build

```

The `install_args` line is the argument string passed to `npm install`. This is optional, and only recommended if your project *needs* to use an install flag.

### `parcel/package.json`

This is a standard `package.json` file. It should contain two entries in the `scripts` section. The `build` script is used during `lektor build -f npm`, and the `watch` script is used during `lektor server -f npm`.

class NPMRunner(object):

    def __init__(self, folder, npm, build_script, watch_script):

    def __init__(self, folder, npm, build_script, watch_script, install_args):

        self.folder = folder

        self.npm = npm

        self.build_script = build_script

        self.watch_script = watch_script

        self.install_args = install_args

    def npm_args(self, *args):

        return popen_args_type([self.npm] + list(args), self.folder)

    def build(self, proc):

        proc.start(self.npm_args('install'), self.npm_args('run', self.build_script))

        proc.start(

            self.npm_args('install', self.install_args),

            self.npm_args('run', self.build_script)

        )

    def watch(self, proc):

        proc.start(self.npm_args('install'), self.npm_args('run', self.watch_script))

                folder=os.path.join(self.env.root_path, section),

                npm=props.get('npm', 'npm'),

                build_script=props.get('build_script', 'build'),

                watch_script=props.get('watch_script', 'watch')

                watch_script=props.get('watch_script', 'watch'),

                install_args=props.get('install_args', '')

            )

    def on_server_spawn(self, extra_flags, **extra):