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.

examples/expected/0051-basic-for-range-bounds.stdout

@@ -0,0 +1,12 @@
+1 2 3 4 | 0<..<5
+0 1 2 3 4 5 | 0=..=5
+1 2 3 4 5 | 0<..=5
+0 1 2 3 4 | 0=..<5
+0 1 2 3 4 | 0..<5
+0 1 2 3 4 5 | 0..=5
+10@3 20@4 30@5 | xs, 2<..
+10@5 20@6 30@7 | xs, 5=..
+inline 0<..=3 sum=6
+2 3 4 | lo<..=hi
+cmp ok
+shl=6