feat: add Unicode conversion support

[qazxcdswe123]

• String to utf8 bytes
• utf8 bytes to String

TODO: use buffer?

[peter-jerry-ye-code-review]

Observations and Suggestions

1. Potential Bug in UTF-8 Encoding Function (to_utf8):

   • The function to_utf8 assumes that the input string is in UTF-16 LE format but does not explicitly check or convert the input string from other encodings. This could lead to incorrect encoding if the input string is not in UTF-16 LE format.
   • Suggestion: Add a validation or conversion step to ensure the input string is in UTF-16 LE format before processing.

2. Potential Bug in UTF-8 Decoding Function (from_utf8):

   • The function from_utf8 assumes that the input byte sequence is a valid UTF-8 encoded string. If the input contains invalid UTF-8 sequences, the function will fail with an error.
   • Suggestion: Consider adding more robust error handling or recovery mechanisms to handle invalid UTF-8 sequences gracefully, possibly by replacing or skipping invalid bytes.

3. Typo in Test Case Description:

   • In the test case "from utf8", the description mentions "from utf8" but the actual function call uses the correct naming convention (@unicode.from_utf8!).
   • Suggestion: Correct the description in the test case to match the function name for clarity.

These observations highlight potential areas for improvement to ensure the functions handle various input scenarios more robustly and adhere to coding standards for clarity and consistency.

[peter-jerry-ye]

Goal or non-goal:

• streaming conversion using Iter

  fn to_utf8(string: Iter[Char]) -> Iter[Byte]!Failure
  

• streaming conversion of segmented data

  let decoder = Decoder::new()
  inspect!(decoder.decode([0xf0]), content="")
  inspect!(decoder.decode([0x9f, 0x98, 0x90]), content="😐")
  

[peter-jerry-ye]

Closed as in favor of moonbitlang/core#1236