fix(cbor): correctly size object keys with multi-byte characters

[eeshsaxena]

Closes #7255.

encodeCbor() throws on objects whose keys contain multi-byte UTF-8 characters:

import { encodeCbor } from "@std/cbor";

encodeCbor({ "é☃é☃é☃é☃": 1 });
// RangeError: offset is out of bounds

calcEncodingSize() sizes object keys as y.length bytes, but keys are written as UTF-8 by encodeString(), which can use up to 3 bytes per UTF-16 code unit. String values already account for this (x.length * 3); keys did not, so the buffer was under-allocated and the write ran off the end.

Sized keys as y.length * 3 to match the string-value branch. Thanks to @bartlomieju for pinpointing the cause in the #7255 discussion.

Added a test mirroring the existing string-preallocation test, using a multi-byte key. It fails on main (RangeError) and passes with the change.

deno test cbor/encode_cbor_test.ts
# 25 passed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (74f08b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7287      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      617              
  Lines       51637    51638       +1     
  Branches     9359     9359              
==========================================
  Hits        49075    49075              
  Misses       2021     2021              
- Partials      541      542       +1     

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