This PR does two things:

1. I moved `package.json` into the root directory. Why? We are using `npx` from the `Makefile` which is executing usually under the root directory. If we install `node_modules` in `src/js` then these invocations don't see `node_modules` and install an extra copy of the packages. So we want `node_modules` in the root directory. node wants to put `node_modules` in the same directory as `package.json` hence I moved `package.json`. I also updated the build rules so that any recipe that uses `npx` has a dependency on `node_modules/.installed`.
2. We invoke `file_packager.py` from both the Makefile (in two places) and from `build_pkg`. There has been some drift in which options are applied in the two places. I extracted the common options and the task of locating the `file_packager.py` script into `tools/file_packager.sh`. I also shortened the make rules for `distutils.data` and `test.data` and made some minor updates to the dev docs about `file_packager.py`.
MAINT Improvements to node and file_packager invocations. This PR does two things:

1. I moved `package.json` into the root directory. Why? We are using `npx` from the `Makefile` which is executing usually under the root directory. If we install `node_modules` in `src/js` then these invocations don't see `node_modules` and install an extra copy of the packages. So we want `node_modules` in the root directory. node wants to put `node_modules` in the same directory as `package.json` hence I moved `package.json`. I also updated the build rules so that any recipe that uses `npx` has a dependency on `node_modules/.installed`.
2. We invoke `file_packager.py` from both the Makefile (in two places) and from `build_pkg`. There has been some drift in which options are applied in the two places. I extracted the common options and the task of locating the `file_packager.py` script into `tools/file_packager.sh`. I also shortened the make rules for `distutils.data` and `test.data` and made some minor updates to the dev docs about `file_packager.py`.
from pyodide_build.testing import set_webdriver_script_timeout


class _MockDriver:
    def set_script_timeout(self, value):
        self._timeout = value


class _MockSelenium:
    script_timeout = 2
    driver = _MockDriver()


def test_set_webdriver_script_timeout():
    selenium = _MockSelenium()
    assert not hasattr(selenium.driver, "_timeout")
    with set_webdriver_script_timeout(selenium, script_timeout=10):
        assert selenium.driver._timeout == 10
    assert selenium.driver._timeout == 2

from pyodide_build.testing import set_webdriver_script_timeout


class _MockDriver:
    def set_script_timeout(self, value):
        self._timeout = value


class _MockSelenium:
    script_timeout = 2
    driver = _MockDriver()


def test_set_webdriver_script_timeout():
    selenium = _MockSelenium()
    assert not hasattr(selenium.driver, "_timeout")
    with set_webdriver_script_timeout(selenium, script_timeout=10):
        assert selenium.driver._timeout == 10
    assert selenium.driver._timeout == 2
