Support binding JS strings to [:0]const u8

Some APIs need a null-terminated string. Currently, they have to ask for a
`[]const u8` and then convert it to a `[:0]const u8`. This is 2 allocations: 1
for jsruntime to get the `[]const u8` from v8, and then one to get the [:0]. By
supporting `[:0]const u8` directly, this is now a single allocation.

src/runtime/js.zig

@@ -1862,7 +1862,13 @@ fn Caller(comptime E: type) type {
                     },
                     .slice => {
                         if (ptr.child == u8) {
-                            return valueToString(self.call_allocator, js_value, self.isolate, self.context);
+                            if (ptr.sentinel()) |s| {
+                                if (comptime s == 0) {
+                                    return valueToStringZ(self.call_allocator, js_value, self.isolate, self.context);
+                                }
+                            } else {
+                                return valueToString(self.call_allocator, js_value, self.isolate, self.context);
+                            }
                         }
 
                         // TODO: TypedArray
@@ -2241,6 +2247,16 @@ fn valueToString(allocator: Allocator, value: v8.Value, isolate: v8.Isolate, con
     return buf;
 }
 
+fn valueToStringZ(allocator: Allocator, value: v8.Value, isolate: v8.Isolate, context: v8.Context) ![:0]u8 {
+    const str = try value.toString(context);
+    const len = str.lenUtf8(isolate);
+    const buf = try allocator.alloc(u8, len + 1);
+    const n = str.writeUtf8(isolate, buf[0..len]);
+    std.debug.assert(n == len);
+    buf[len] = 0;
+    return buf[0..len :0];
+}
+
 const NoopInspector = struct {
     pub fn onInspectorResponse(_: *anyopaque, _: u32, _: []const u8) void {}
     pub fn onInspectorEvent(_: *anyopaque, _: []const u8) void {}

src/runtime/test_primitive_types.zig

@@ -96,6 +96,14 @@ const Primitives = struct {
     pub fn _checkOptionalReturnString(_: *const Primitives) ?[]const u8 {
         return "ok";
     }
+
+    pub fn _echoString(_: *const Primitives, a: []const u8) []const u8 {
+        return a;
+    }
+
+    pub fn _echoStringZ(_: *const Primitives, a: [:0]const u8) []const u8 {
+        return a;
+    }
 };
 
 const testing = @import("testing.zig");
@@ -172,5 +180,9 @@ test "JS: primitive types" {
         .{ "p.checkOptionalReturn() === true;", "true" },
         .{ "p.checkOptionalReturnNull() === null;", "true" },
         .{ "p.checkOptionalReturnString() === 'ok';", "true" },
+
+        // strings
+        .{ "p.echoString('over 9000!');", "over 9000!" },
+        .{ "p.echoStringZ('Teg');", "Teg" },
     }, .{});
 }