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.

src/token.zig

@@ -53,6 +53,13 @@ pub const Tag = enum {
     dot, // .
     dot_dot, // ..
     dot_dot_eq, // ..=
+    dot_dot_lt, // ..<
+    lt_dot_dot, // <..
+    lt_dot_dot_eq, // <..=
+    lt_dot_dot_lt, // <..<
+    eq_dot_dot, // =..
+    eq_dot_dot_eq, // =..=
+    eq_dot_dot_lt, // =..<
     dollar, // $
 
     // Operators
@@ -152,6 +159,13 @@ pub const Tag = enum {
             .dot => ".",
             .dot_dot => "..",
             .dot_dot_eq => "..=",
+            .dot_dot_lt => "..<",
+            .lt_dot_dot => "<..",
+            .lt_dot_dot_eq => "<..=",
+            .lt_dot_dot_lt => "<..<",
+            .eq_dot_dot => "=..",
+            .eq_dot_dot_eq => "=..=",
+            .eq_dot_dot_lt => "=..<",
             .dollar => "$",
             .plus => "+",
             .minus => "-",