update(Modal): Add optional zIndex to props

[nilgradisnik]

to: @williaster @alecklandgraf

Description

Allow for optional zIndex to be passed to Modal.

Motivation and Context

We want to override z-index for Modals that are in a Lighbox component where a recent change increased the z-index #411. This allows to bring such Modals to the front.

Testing

Tested in storybook

Screenshots

Checklist

• My code follows the style guide of this project.
• I have updated or added documentation accordingly.
• I have read the CONTRIBUTING document.

[airbnb-bot]

Size Changes

Package	Diff	ESM	Prev ESM	CJS	Prev CJS
core	+0.0%	570.87 KB	570.69 KB	715.98 KB	715.76 KB

Compared to master. File sizes are unminified and ungzipped.

View raw build stats

Previous (master)

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    "lib": 14147
  },
  "app-shell": {
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  },
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    "lib": 732939
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  "forms": {
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    "lib": 49298
  },
  "icons": {
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    "lib": 205626
  },
  "layouts": {
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    "lib": 20770
  },
  "metrics": {
    "esm": 5467,
    "lib": 7729
  },
  "test-utils": {
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    "lib": 5937
  }
}

Current

{
  "apollo": {
    "esm": 10832,
    "lib": 14147
  },
  "app-shell": {
    "esm": 12906,
    "lib": 19874
  },
  "composer": {
    "esm": 68247,
    "lib": 101805
  },
  "core": {
    "esm": 584567,
    "lib": 733164
  },
  "forms": {
    "esm": 37350,
    "lib": 49298
  },
  "icons": {
    "esm": 156355,
    "lib": 205626
  },
  "layouts": {
    "esm": 15298,
    "lib": 20770
  },
  "metrics": {
    "esm": 5467,
    "lib": 7729
  },
  "test-utils": {
    "esm": 4279,
    "lib": 5937
  }
}

[alecklandgraf]

what's the difference between auto and Z_INDEX_MODAL here?

[nilgradisnik]

This is saying zIndex can be either a number or a string auto. This is a typescript type union, not a default value.