lang: slice ranges take the same bound markers as for-header ranges

xs[1..=3] (end inclusive), xs[0<..<4] (both exclusive), xs[..=2]
(prefix form with markers, implicit 0 start), xs[2<..] (open end,
exclusive start), and xs[..] (whole collection) — lowered as lo+1 /
hi+1 on the existing subslice op. Strings slice through the same path.
An explicit end marker requires an end expression, matching the
for-header rule.

Regression: examples/0052-basic-slice-range-bounds.sx.

src/ast.zig

@@ -594,8 +594,12 @@ pub const IndexExpr = struct {
 
 pub const SliceExpr = struct {
     object: *Node,
-    start: ?*Node = null,
-    end: ?*Node = null,
+    start: ?*Node = null, // null = 0
+    end: ?*Node = null, // null = len
+    /// `<..` family — slice begins one past `start`.
+    start_exclusive: bool = false,
+    /// `..=` family — slice includes `end`.
+    end_inclusive: bool = false,
 };
 
 pub const PointerTypeExpr = struct {