lang: range bound markers — '=' inclusive / '<' exclusive on either side of '..'

Each side of '..' takes an optional bound marker, defaulting to
start-inclusive, end-exclusive (a..b == a=..<b; a..=b stays the short
end-inclusive spelling):

    for 0<..<N (i) { }   // 1 .. N-1   (both exclusive)
    for 0=..=N (i) { }   // 0 .. N     (both inclusive)
    for 0<..=N (i) { }   // 1 .. N
    for 0..<N  (i) { }   // 0 .. N-1   (explicit default)
    for xs, 2<.. (x, i)  // open range, exclusive start: i = 3, 4, ...

The nine lexemes are single tokens (maximal munch on '<'/'='/'..'), so
expression parsing never sees the leading marker as a comparison; '<',
'<<', '<=', '==', '=>' lex unchanged. An explicit end marker makes the
end expression mandatory; open forms are a.. / a<.. / a=... Works in
runtime, multi-iterable, and inline-for headers.

Regression: examples/0051-basic-for-range-bounds.sx (full matrix, open
start-marked ranges, comptime unroll, runtime bounds, lexer
non-regression); 1152's pinned message generalized.

readme.md

@@ -345,6 +345,8 @@ for items (val) { print("{}\n", val); }
 for items, 0.. (val, idx) { print("[{}] = {}\n", idx, val); }
 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
 
 // Defer
 f := open("file.txt");