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.

examples/expected/0052-basic-slice-range-bounds.stderr

@@ -0,0 +1 @@
+