tracking: JSValue safety in our Zig and JS bindings

[nektro]

• better code generator
• rid nearly all use of ExceptionRef in zig #15100
• allow zig js host functions to return JSError #15120
• cpp: Bun::toStringRef: return dead when exception has been thrown #15127
• zig: remove JSValue.isEmpty #15128
• replace orelse return null with try
• replace catch return null with try (done in 15120)
• make JSValue.get() return a possible JSError
• make JSValue.toZigString() return a possible JSError
• make globalThis.throw* return a JSError
• when done, audit all globalThis.hasException() calls for redundancy
• audit all fromJS() functions to return possible JSError
• make globalThis.ERR_*().throw() return a JSError

more to come