chore: move configurations under `package.json`

[SauravMaheshkar]

Fixes #302

Similar to https://github.com/jupyter-server/jupyter-resource-usage/pull/188 and https://github.com/jupyter-server/jupyter_server/pull/1246

Changes proposed by this PR can be summarised as follows :-

• Move the content from .prettierrc under the prettier key in package.json. Source(prettier docs)
• Move the content from .eslintrc.js under the eslintConfig key in package.json. Source(eslint docs)
• Move the content from .eslintignore under the eslintIgnore key in package.json. Source(eslint docs)
• Move the content from .stylelintrc under the stylelint key in package.json. Source(stylelint docs)

Doing so, would lead to a more minimal project structure without any functional change.

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[fcollonval]

@meeseeksdev please backport to 4.0