parser: start the arduous journey of hooking up diagnostics

The errors in the line buffer and tokenizer now have diagnostics. The
line number is trivial to keep track of due to the line buffer, but
the column index requires quite a bit of juggling, as we pass
successively trimmed down buffers to the internals of the parser.
There will probably be some column index counting problems in the
future. Also, handling the diagnostics is a bit awkward, since it's a
mandatory out-parameter of the parse functions now. The user must
provide a valid diagnostics object that survives for the life of the
parser.

src/parser/state.zig

@@ -4,6 +4,7 @@ const tokenizer = @import("../tokenizer.zig");
 const Error = @import("../parser.zig").Error;
 const DuplicateKeyBehavior = @import("../parser.zig").DuplicateKeyBehavior;
 const Options = @import("../parser.zig").Options;
+const Diagnostics = @import("../parser.zig").Diagnostics;
 const Value = @import("./value.zig").Value;
 
 pub const Document = struct {
@@ -42,14 +43,16 @@ pub const State = struct {
     pub const Stack = std.ArrayList(*Value);
 
     document: Document,
+    diagnostics: *Diagnostics,
     value_stack: Stack,
     mode: enum { initial, value, done } = .initial,
     expect_shift: tokenizer.ShiftDirection = .none,
     dangling_key: ?[]const u8 = null,
 
-    pub fn init(allocator: std.mem.Allocator) State {
+    pub fn init(allocator: std.mem.Allocator, diagnostics: *Diagnostics) State {
         return .{
             .document = Document.init(allocator),
+            .diagnostics = diagnostics,
             .value_stack = Stack.init(allocator),
         };
     }