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.

readme.md

@@ -347,6 +347,7 @@ for 1..=5, 0.. (a, b) { print("{}:{}\n", a, b); }   // a: 1..5, b follows
 for items (val) => total += val;                    // arrow body
 for 0<..<n (i) { }                                  // bound markers: 1 .. n-1
 for 0=..=n (i) { }                                  //                0 .. n
+sub := items[1..=3];                                // slices take them too
 
 // Defer
 f := open("file.txt");