encoding/json/v2: reject unquoted dash as a JSON field name

In this blog:

	https://blog.trailofbits.com/2025/06/17/unexpected-security-footguns-in-gos-parsers/

the concern was raised that whenever "-" is combined with other options,
the "-" is intepreted as as a name, rather than an ignored field,
which may go contrary to user expectation.

Static analysis demonstrates that there are ~2k instances of `json:"-,omitempty"
in the wild, where almost all of them intended for the field to be ignored.

To prevent this footgun, reject any tags that has "-," as a prefix
and warn the user to choose one of the reasonable alternatives.

The documentation of json/v2 already suggests `json:"'-'"`
as the recommended way to explicitly specify dash as the name.
See Example_fieldNames for example usages of the single-quoted literal.

Update the v1 json documentation to suggest the same thing.

Updates #71497

Change-Id: I7687b6eecdf82a5d894d057c78a4a90af4f5a6e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/683175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/encoding/json/v2_decode_test.go

@@ -1195,6 +1195,27 @@ var unmarshalTests = []struct {
 		out:      []int{1, 2, 0, 4, 5},
 		err:      &UnmarshalTypeError{Value: "bool", Type: reflect.TypeFor[int](), Field: "2", Offset: len64(`[1,2,`)},
 	},
+
+	{
+		CaseName: Name("DashComma"),
+		in:       `{"-":"hello"}`,
+		ptr: new(struct {
+			F string `json:"-,"`
+		}),
+		out: struct {
+			F string `json:"-,"`
+		}{"hello"},
+	},
+	{
+		CaseName: Name("DashCommaOmitEmpty"),
+		in:       `{"-":"hello"}`,
+		ptr: new(struct {
+			F string `json:"-,omitempty"`
+		}),
+		out: struct {
+			F string `json:"-,omitempty"`
+		}{"hello"},
+	},
 }
 
 func TestMarshal(t *testing.T) {