Merge ScriptStencilBase, ScriptStencil, and FunctionStencil, and split bytecode into ImmutableScriptData (fixes #554)

[arai-a]

• ScriptStencilBase and ScriptStencil are merged into ScriptStencil
• ScriptStencil is the struct for top-level script, non-lazy function, and lazy function
• ImmutableScriptData holds bytecode etc that is owned by non-lazy script (both top-level and function)
• FunctionStencil was jsparagus-specific struct that has some field of FunctionBox, to instantiate JSFunction. those fields are going to be moved into ScriptStencil in m-c, so we move it into ScriptStencil here as well
• Vec≤RegExp> is shared across all scripts, so moved to EmitResult
• EmitResult now contains top level script ScriptStencil, and Vec<ScriptStencil> for functions, and Vec<ImmutableScriptData> for non-lazy scripts

[arai-a]

smoosh part in https://bugzilla.mozilla.org/show_bug.cgi?id=1644698