Add missing "react-dom/server.bun" entry in package.json "exports" (#26402)

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## Summary

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`react-dom/server` in Bun (correctly) chooses `react-dom/server.bun`,
but `react-dom/server.bun` currently can't be imported because it is not
included in package.json `"exports"` (`react-dom/server` works,
`react-dom/server.bun` doesn't). Previously, I didn't think it was
necessary to do that, but it is too easy to accidentally run the browser
build in unit tests when importing `react-dom/server`

This also aligns behavior of package.json `"exports"` of
`react-dom/server.bun` with `react-dom/server.browser`,
`react-dom/server.node`, and the rest.

## How did you test this change?

Manually edited package.json in node_modules in a separate folder and
ran tests in Bun with `react-dom/server.bun` as the import specifier

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Author: Jarred-Sumner

packages/react-dom/package.json

@@ -58,6 +58,7 @@
       "default": "./server.node.js"
     },
     "./server.browser": "./server.browser.js",
+    "./server.bun": "./server.bun.js",
     "./server.edge": "./server.edge.js",
     "./server.node": "./server.node.js",
     "./static": {