fix: fix pnpm start script on the root directory

[shabaraba]

Why

Trying the command, pnpm run start, on the root directory, the error is faced as below:

packages/plugin-packer postbuild: ERROR: Invalid Option: --watch
packages/rest-api-client postbuild: ERROR: Invalid Option: --watch
packages/plugin-packer postbuild: Failed
js-sdk/packages/plugin-packer:
 ERR_PNPM_RECURSIVE_RUN_FIRST_FAIL  @kintone/plugin-packer@8.1.3 postbuild: `run-p js css "--watch"`
Exit status 1
packages/rest-api-client postbuild: Failed

What

Problem:
When the root start script (pnpm build --watch) was executed, pnpm prebuild && tsc --build packages --force && pnpm postbuild "--watch" was actually executed.
The postbuild script for rest-api-client package and plugin-packer package did not support the "--watch" option.

Solution:
Because children packages have each start script, modify the start script for root package.json to run all child package start scripts in parallel.

How to test

run pnpm start on the root directory

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added tests if it is required.
• Passed pnpm lint and pnpm test on the root directory.

[tasshi-me]

LGTM!