Test for WebKit based browsers instead of Safari

[josephmr]

Description
The previous IS_SAFARI test did not catch some WebKit based environments that should have the special cased behavior. For example, running Slate inside a webview on iOS would result in a userAgent like: "Mozilla/5.0 (iPhone; CPU iPhone OS 16_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148" which would not match the previous IS_SAFARI definition, but does have the same quirks since it is WebKit based.

Issue
No matching issue reported.

Example
One manifestation of this is the special behavior for Chrome/Safari (i.e. WebKit) for handling deleting selected inline voids relevant code.

Prior to this change in a webview on iOS it would fail to delete a selected inline void since this special cased behavior wouldn't fire -- in these videos the input is a webview inside a react-native application; the webview is then using Slate and "design-triage" is an inline void element:

Screen.Recording.2023-05-30.at.10.59.40.AM.mov

After this change it deletes as expected:

Screen.Recording.2023-05-30.at.10.56.35.AM.mov

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 602e8c7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Minor

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