Skip the indentation after a newline 8 bytes at a time (#5810)

Pretty-printed JSON indents every line with a run of spaces, which
skip_whitespace() read one character at a time. For contiguous input
(bulk_scan) the lexer now skips the spaces after a newline 8 bytes at a
time directly in the input buffer, including the leading spaces of the
last, partial word. This bypasses get(), which is safe because
bulk_scan implies the lazy position from the previous commit: nothing
is counted per character.

Compared to lazy positions alone, pretty-printed input (twitter,
citm_catalog with indent 2 and 4) parses up to 13% faster and accept()
gets up to 30% faster (GCC 13 and clang 18 on x86-64, Apple clang 21 on
arm64). Minified input is unchanged within noise.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-class_lexer.cpp

@@ -938,6 +938,10 @@ TEST_CASE("Eisel-Lemire float conversion")
             const int k = i % 64;
             const std::uint64_t x = (std::uint64_t{1} << k) | (a & ((std::uint64_t{1} << k) - 1));
             CHECK(nlohmann::detail::count_leading_zeros(x) == 63 - k);
+
+            // the mirror image: bit k set, arbitrary bits above it
+            const std::uint64_t y = (std::uint64_t{1} << k) | (k == 63 ? 0 : (a << (k + 1)));
+            CHECK(nlohmann::detail::count_trailing_zeros(y) == k);
         }
     }