Fix ID generation to ensure it produces valid CSS identifiers

[alrra]

Description

• Move the logic of generating the ID into its own utility function.

• Ensure the utility function generates IDs that are valid CSS identifiers.

  shortid.generate() by itself will generates IDs that are not always valid ¹ CSS identifiers (e.g. will start with a digit). So, when other parts of the codebase will use those IDs as a selector, a SyntaxError ² will be thrown (i.e. '#...' is not a valid selector).

  The slds- prefix is used so to keep things simple. shortid.characters() requires "a string of all 64 unique characters" ³, which means that in order to generate valid CSS identifiers a-z, A-Z, and some "ISO 10646 characters U+00A0 and higher" ¹ will need to be included.

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CONTRIBUTOR checklist (do not remove)

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• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.

REVIEWER checklist (do not remove)

• CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.

Required only if there are markup / UX changes

• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
• While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.

Footnotes

1. From https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier
   In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit. " ↩ ↩²

2. https://dom.spec.whatwg.org/#selectors ↩

3. https://github.com/dylang/shortid/blob/937ce2c8dd7001baec1785cb8dce6e6fe1bcf61f/README.md#shortidcharactersstring ↩

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@alrra Thank you for your contribution!

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Congrats on merging your first pull request to Design System React! 🎉
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