enabled built-in go error formatting instead of requriring tintErr wrapping, colorized debug level text to increase readability

[pwntr]

Thank you this nice package, since moving to slog I was missing the pretty zerolog-like output. Until I discovered tint :)!

PR summary:

• bugfix/feature [45f85b4]: without resorting to the package provided tintErr error type wrapper, users using the native slog API to log errors didn't get colorized/red output for error messages.
  By asserting the built-in error interface directly, both the tintErr and any other errors (satisfying the error interface) get the colorization treatment, also regardless of the key provided for the error value.
  This kinda makes the whole tintError type obsolete (and I would recommend removing it altogether eventually), but I left it in for backwards compatibility in case users of this package called tint.Err(err) in their business logic or own package wrappers. Using the native slog API is certainly more portable, as one doesn't need to provide a wrapper package for slog and/or tint in that case.

  Preview:
  

• feature [3fbd1c2]: added faint magenta as a foreground color for debug messages, to visually isolate the DBG level text better from the log message text that follows -> increases readability when glancing over output and increases 🌈 level

  Preview:
  

[lmittmann]

I appreciate the effort, but will not merge this PR.

bugfix/feature [https://github.com/lmittmann/tint/commit/45f85b4332dd6db4f3428d8b6dd319edde5bd961]

I consider it a feature to be able to only log some errors in color. Since #66 you can change the tint.Err key value, if needed.

feature [https://github.com/lmittmann/tint/commit/3fbd1c23bc3bd4c79d4c68e630b88a09dc777416]

If you want to customize the level color you can use the ReplaceAttr-option. Although I admit, this is not very pleasant. However, a better API to customize the level color without hurting performance has not been proposed yet.