[wpiutil] Refactor FileLogger

[Gold856]

FileLogger now logs entire flushed blocks of data, instead of breaking it up into lines.

Fixes #7135.

[Gold856]

/format

[KangarooKoala]

Do we need to clarify the LineBuffer() doc comment? Here's what it says for reference:

  /**
   * Creates a function that chunks incoming data into lines before calling the
   * callback with the individual line.
   *
   * @param callback The callback that logs lines.
   * @return The function.
   */
  static std::function<void(std::string_view)> LineBuffer(
      std::function<void(std::string_view)> callback);

I think the wording is ambiguous enough that it could be interpreted as either the old behavior or the new behavior, though I'm not sure if that's a reason for or against changing it.

[Gold856]

/format

[KangarooKoala]

Overall, looks good! Maybe it's just me, but I'm still iffy about the doc comment wording- "Blocks of whole lines" could be interpreted as "blocks that are split at line endings" or as "blocks that each consist of a whole line". I think it's probably good enough for this PR to be merged- I don't think many people will look at Buffer() anyways, especially since it's C++ (and by extension Python) only- but if anyone has any good ideas then maybe we could discuss those before merging this.

[rzblue]

I think this is good to merge for beta 1