Scripts: #strict on runs the script with strict write checks instead of refusing it

`#strict on` and `//@strict on` no longer end the run as unsupported. They
turn on the checks the dollar-object help page gives for `$.strict`
(https://extendscript.docsforadobe.dev/extendscript-tools-features/dollar-object/):
an assignment to a read-only property throws a TypeError, and so does a new
property on an object that takes none.

VectorCraft's own rules: a getter-only property counts as read-only; the
objects that take no new properties are the non-extensible ones
(preventExtensions, seal, freeze, and the frozen vc and $ objects); the last
`#strict` line of the script and its includes sets the whole run, and runs
without one are unchanged; eval and Function take the checks of the code
that compiles them; a file $.evalFile runs keeps its own line's checks for
its code and functions, or takes the caller's without one. The rest of ES5
strict mode stays with 'use strict'.

The js crate carries the setting per compiled source
(Realm::set_strict_writes, and a strict_writes callback on
HostItem::Evaluator), and applies it only to property assignments.

crates/js/src/bytecode.rs

@@ -173,6 +173,9 @@ pub(crate) struct Source {
     pub(crate) text: Rc<[u16]>,
     /// Which source this is, as [`crate::Realm::error_source`] gives it.
     pub(crate) tag: u32,
+    /// Writes to read-only and getter-only properties, and new properties on objects that are
+    /// not extensible, throw in this source's code even where it is not strict mode code.
+    pub(crate) strict_writes: bool,
 }
 
 #[derive(Debug)]