Upgrade WebKit

docs/upgrading-webkit.md

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+# Upgrading WebKit
+
+Bun uses [a fork](https://github.com/Jarred-Sumner/WebKit) of WebKit with a small number of changes.
+
+It's important to periodically update WebKit for many reasons:
+
+- Security
+- Performance
+- Compatibility
+- …and many more.
+
+To upgrade, first find the commit in **bun's WebKit fork** (not bun!) between when we last upgraded and now.
+
+```bash
+cd src/javascript/jsc/WebKit # In the WebKit directory! not bun
+git checkout $COMMIT
+```
+
+This is the main command to run:
+
+```bash
+git pull https://github.com/WebKit/WebKit.git main --no-rebase --allow-unrelated-histories -X theirs
+```
+
+Then, you will likely see some silly merge conflicts. Fix them and then run:
+
+```bash
+# You might have to run this multiple times.
+rm -rf WebKitBuild
+
+# Go to Bun's directory! Not WebKit.
+cd ../../../../
+make jsc-build-mac-compile
+```
+
+Make sure that JSC's CLI is able to load successfully. This verifies that the build is working.
+
+You know this worked when it printed help options. If it complains about symbols, crashes, or anything else that looks wrong, something is wrong.
+
+```bash
+src/javascript/jsc/WebKit/WebKitBuild/Release/bin/jsc --help
+```
+
+Then, clear out our bindings and regenerate the C++<>Zig headers:
+
+```bash
+make clean-bindings jsc-bindings-headers generate-builtins
+```
+
+Now update Bun's bindings wherever there are compiler errors:
+
+```bash
+# It will take awhile if you don't pass -j here
+make jsc-bindings-mac -j10
+```
+
+This is the hard part. It might involve digging through WebKit's commit history to figure out what changed and why. Fortunately, WebKit contributors write great commit messages.