fix: Don't free borrowed string slices in Value.deinit

[pepicrft]

Summary

The initString function stores borrowed slices, not owned copies. Previously, deinit tried to free these slices, causing crashes when they pointed to string literals, template source, or user data.

This change makes deinit only free container memory (arrays, objects) while leaving string memory management to the caller.

The Problem

var context = Value.initObject(allocator);
try context.object.put("title", Value.initString("Hello"));  // borrowed slice!
context.deinit(allocator);  // CRASH: tries to free "Hello" string literal

The Solution

Users who need owned strings should use allocator.dupe() and manage cleanup with arena allocators:

var arena = std.heap.ArenaAllocator.init(allocator);
defer arena.deinit();

var context = Value.initObject(arena.allocator());
try context.object.put("title", Value.initString(try arena.allocator().dupe(u8, title)));
// No need to call context.deinit() - arena handles everything