Add Page.Contents with scope support

Note that this also adds a new `.ContentWithoutSummary` method, and to do that we had to unify the different summary types:

Both `auto` and `manual` now returns HTML. Before this commit, `auto` would return plain text. This could be considered to be a slightly breaking change, but for the better: Now you can treat the `.Summary` the same without thinking about where it comes from, and if you want plain text, pipe it into `{{ .Summary | plainify }}`.

Fixes #8680
Fixes #12761
Fixes #12778
Fixes #716

common/types/types.go

@@ -107,12 +107,20 @@ func Unwrapv(v any) any {
 	return v
 }
 
-// LowHigh is typically used to represent a slice boundary.
-type LowHigh struct {
+// LowHigh represents a byte or slice boundary.
+type LowHigh[S ~[]byte | string] struct {
 	Low  int
 	High int
 }
 
+func (l LowHigh[S]) IsZero() bool {
+	return l.Low < 0 || (l.Low == 0 && l.High == 0)
+}
+
+func (l LowHigh[S]) Value(source S) S {
+	return source[l.Low:l.High]
+}
+
 // This is only used for debugging purposes.
 var InvocationCounter atomic.Int64
 

common/types/types_test.go

@@ -27,3 +27,25 @@ func TestKeyValues(t *testing.T) {
 	c.Assert(kv.KeyString(), qt.Equals, "key")
 	c.Assert(kv.Values, qt.DeepEquals, []any{"a1", "a2"})
 }
+
+func TestLowHigh(t *testing.T) {
+	c := qt.New(t)
+
+	lh := LowHigh[string]{
+		Low:  2,
+		High: 10,
+	}
+
+	s := "abcdefghijklmnopqrstuvwxyz"
+	c.Assert(lh.IsZero(), qt.IsFalse)
+	c.Assert(lh.Value(s), qt.Equals, "cdefghij")
+
+	lhb := LowHigh[[]byte]{
+		Low:  2,
+		High: 10,
+	}
+
+	sb := []byte(s)
+	c.Assert(lhb.IsZero(), qt.IsFalse)
+	c.Assert(lhb.Value(sb), qt.DeepEquals, []byte("cdefghij"))
+}