fix: Support decoding N-dimensional arrays

[nebkat]

Fixes a decoding gap: N-dimensional arrays are a draft 3 construct that this library
rejected as invalid, so valid draft 3 data could not be read at all.

bjdataDecode([0x5B, 0x24, 0x55, 0x23, 0x5B, 0x55, 2, 0x55, 3, 0x5D, 1, 2, 3, 4, 5, 6]);
// before: FormatException: Invalid BJData marker: 0x5b
// after:  [[1, 2, 3], [4, 5, 6]]

What it does

• [$type#[Nx Ny ...] decodes to nested lists, with the innermost axis kept as the typed
  list, so the spec's 2×3×4 example comes back as a List<List<Uint8List>> matching the
  JSON in the specification verbatim.
• The nesting is built from views, not copies. An N-dimensional array is still one
  contiguous allocation once decoded — a test pins the slices at offsets 0 and 20 into a
  single 24-byte buffer.
• The column-major form is handled too. [$type#[[Nx Ny ...]] is how MATLAB and
  FORTRAN write it, and given JSONLab shares an author with the spec it is likely common
  in real data. Elements are permuted whole, at whatever width, so it reads identically to
  a row-major array of the same shape. Verified against the specification's own
  column-major byte sequence.
• Dimension arrays are accepted in both optimized and non-optimized form; an object
  counted by one is rejected.

Also throws a FormatException rather than a RangeError when a string or buffer runs
past the end of the input, which a payload shorter than its dimensions makes easy to hit.

Not included

Encoding N-dimensional arrays. A decoded array is written back as nested arrays — the
values are unchanged but the format is not. That is added in the follow-up PR, which also
makes the round-trip byte-exact.

Verification

Both worked examples from the specification decode byte-for-byte. 203 tests on the VM and
183 on Chrome, plus dart format, dart analyze --fatal-infos and the examples.

🤖 Generated with Claude Code

[nebkat]

Base of a two-PR stack:

• fix: Support decoding N-dimensional arrays #1 → main — 0.9.3, decoding N-dimensional arrays (this PR)
• feat: Update to BJData draft 4 with Structure-of-Arrays support #2 → nd-array-decoding — 0.10.0, draft 4 and Structure-of-Arrays

Merge this one first; #2 retargets to main automatically afterwards.

🤖 Generated with Claude Code