ESR 91 port

Each commit should contain relevant Bugzilla tickets from Mozilla to document why changes were necessary.

Changes of note:

• Script private values are now associated after compilation (so we have to use the Script/Execute APIs instead of the Evaluate APIs)
• Use of mozilla::Maybe for property descriptors
• GC flag changes
• js:: to JS:: promotion for many string utils
• Top-level await will require some changes to the module loader (and a main loop 😬, it is currently disabled)
• JS_New is removed in favor of JS::Construct

Closes: #413 (closed)