doc: link to Runtime Keys in export conditions

This commit:
1. Adds a link to the WinterCG Runtime Keys proposal draft spec in the
    documentation for the `export` condition.
2. Adds a criteria to add more export conditions to the Node.js docs:
    they should be Node.js core-relevant.
3. Removes the "deno" and "react-native" export conditions from the core
    docs with the expectation that readers will follow the link to the
    Runtime Keys proposal draft spec to learn about them and more.

PR-URL: nodejs#48408
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Jacob Smith <jacob@frende.me>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: jcbhmr

doc/api/packages.md

@@ -704,18 +704,17 @@ is provided below to assist with ecosystem coordination.
 
 * `"types"` - can be used by typing systems to resolve the typing file for
   the given export. _This condition should always be included first._
-* `"deno"` - indicates a variation for the Deno platform.
 * `"browser"` - any web browser environment.
-* `"react-native"` - will be matched by the React Native framework (all
-  platforms). _To target React Native for Web, `"browser"` should be specified
-  before this condition._
 * `"development"` - can be used to define a development-only environment
   entry point, for example to provide additional debugging context such as
   better error messages when running in a development mode. _Must always be
   mutually exclusive with `"production"`._
 * `"production"` - can be used to define a production environment entry
   point. _Must always be mutually exclusive with `"development"`._
 
+For other runtimes, platform-specific condition key definitions are maintained
+by the [WinterCG][] in the [Runtime Keys][] proposal specification.
+
 New conditions definitions may be added to this list by creating a pull request
 to the [Node.js documentation for this section][]. The requirements for listing
 a new condition definition here are that:
@@ -729,6 +728,10 @@ a new condition definition here are that:
   benefit to the ecosystem that wouldn't otherwise be possible. For example,
   this would not necessarily be the case for company-specific or
   application-specific conditions.
+* The condition should be such that a Node.js user would expect it to be in
+  Node.js core documentation. The `"types"` condition is a good example: It
+  doesn't really belong in the [Runtime Keys][] proposal but is a good fit
+  here in the Node.js docs.
 
 The above definitions may be moved to a dedicated conditions registry in due
 course.
@@ -1325,6 +1328,8 @@ This field defines [subpath imports][] for the current package.
 [ES module]: esm.md
 [ES modules]: esm.md
 [Node.js documentation for this section]: https://github.com/nodejs/node/blob/HEAD/doc/api/packages.md#conditions-definitions
+[Runtime Keys]: https://runtime-keys.proposal.wintercg.org/
+[WinterCG]: https://wintercg.org/
 [`"exports"`]: #exports
 [`"imports"`]: #imports
 [`"main"`]: #main