Currently we check if JSScript::function() is nullptr in a number of places to determine if a script is a function script. This requires traversing through several GC edges which makes it annoying to use during finalization or other unusual cases. Instead just define an Immutable flag like we do for the isForEval/isModule checks.

Set this flag for scripts that are for functions. This is equivalent to
checking the kind of the bodyScope() but is safe to call in GC contexts such
as finalization.
Also use an isFunction() accessor instead of checking if JSScript::function()
is nullptr throughout the codebase. Similarly, cleanup similar uses of
JSScript::module().

https://hg.mozilla.org/mozilla-central/rev/821b104d357b