Support JavaScriptCore 6.0

[robertgzr]

• chore: vendor webkit javascriptcore headers
• chore: extend ci matrix to include jobs for jsc-4.0 and jsc-6.0
• Support selecting JavaScriptCore API version

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TODO

• make FindJSC.cmake smart enough to detect headers present in vendor/ and system paths
• install JSC headers as part of includejs bundle

[robertgzr]

@tony-go i guess there was some kind of credit limit for CI on this org/repo?

[robertgzr]

one thing i noticed... we also install FindJavaScriptCore.cmake into the dist dir. that means it will leak our approach to vendor the headers and will not work if used on a system that doesn't also vendor them in the exact same way we do

[jviotti]

Looks good to me

[tony-go]

Asked for a minor change, but looks good

[tony-go]

I wonder if we could not use javascriptcore_version to set DINCLUDEJS_BACKEND_JAVASCRIPTCORE_API_VERSION later.

[robertgzr]

see my latest commit, we could do it like that... but then it ends up being specified (as empty) for non-linux ci runs

[tony-go]

hmm, you're right.

Do you prefer to revert then?

[robertgzr]

i mean it being empty on non-linux isn't really an issue... since the FindJavaScriptCore.cmake just ignores this on macos anyway. although we might want to check that it's not set and warn otherwise

[robertgzr]

any idea why the asan would fail here? i think nothing should have changed

[tony-go]

Hey I already experienced weird stuff with ASAN: https://bugs.webkit.org/show_bug.cgi?id=269791

[robertgzr]

left some TODOs in the PR description