chore: Export ETHERSCAN_SUPPORTED_CHAIN_IDS constant from Preferences…

… Controller (#4233)

## Explanation

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your changes:

* What is the current state of things and why does it need to change?
Currently ETHERSCAN_SUPPPORTED_CHAIN_IDS is not allowed to be accessed
by consumers of PreferencesController
* What is the solution your changes offer and how does it work?
Allow ETHERSCAN_SUPPORTED_CHAIN_IDS be imported from consumers of
PreferencesController

* If you had to upgrade a dependency, why did you do so?
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## References

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* Fixes #12345
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## Changelog

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### `@metamask/preferences-controller`

- **<ADDED>**: export of ETHERSCAN_SUPPORTED_CHAIN_IDS for clients

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

packages/preferences-controller/src/index.ts

@@ -1 +1,2 @@
 export * from './PreferencesController';
+export { ETHERSCAN_SUPPORTED_CHAIN_IDS } from './constants';