Development mode improvments

[ashb]

Hello, I've just come across this library and it is almost perfect for my needs, but I have a few features I'd like to add.

For now I'll list them all here, in my rough priority order, and I'm happy to implement them myself as PRs if you like the features

• Add an exported func to be able to set a new level color for Development mode #3 Customized colors/add colors for new log levels

  Although slog doesn't have a TRACE level (lower than debug) the project I am integrating with uses hashicorp's hclog which does, and I'd like this level to be colored differently to DEBUG.

• Space around keys in dev mode

  Rather that this:

  2025-09-25T10:57:10.232+01:00  INFO     test    {"logger":"test","time":"abc","http":{"other":"value"}}
  

  I'd like it to look something like this

  2025-09-25T10:57:10.232+01:00  INFO     test    {"logger": "test", "time": "abc", "http":{ "other": "value" }}
  

• Non-JSON mode for attrs in dev mode

  This one I'm less set on/wouldn't be that upset if you said no to, but I'd like the ability for a non JSON mode too:

  2025-09-25T10:57:10.232+01:00  INFO     test    logger=test time=abc http.other=value
  

Thoughts? For 2 and 3 they would be new options in the Config struct, 1 could be there, or could be a global thing.

• Set/override names for custom logging levels

  An extension to the above, when using a level that isn't one of the standard levels from slog.Level, it would be nice to be able to set the level name. For example if I have:

  LevelTrace = slog.LevelDebug  -4

  This will print it as either DEBUG-4 or {"level":"DEBUG-4"} which is not all that helpful. (This applies to both development and non developmnent modes.)

[icefed]

Thank you for suggestions.

Customized colors/add colors for new log levels
Although slog doesn't have a TRACE level (lower than debug) the project I am integrating with uses hashicorp's hclog which does, and I'd like this level to be colored differently to DEBUG.

It's easy to add color for TRACE level, but for Customized colors will need a exported API to do this, maybe all level's color can be customized.

Space around keys in dev mode

Golang have two styles for marshaling json, the Marshal for compact and MarshalIndent for pretty, in Indent style there is a space after ':' ex ': '. Add a space after ':' ',' is acceptable for me, space around keys makes output strange.

Space around keys

{ "logger": "test", "time": "abc", "http": { "other": "value"}}

Add a space after ':' ',', I though you want this

{"logger": "test", "time": "abc", "http": {"other": "value"}}

Non-JSON mode for attrs in dev mode

The Logger arguments only receive JSONHandler which is for JSON encoding output, also JSONHandler support dev mode formatting to text just for buildin keys.
We should implement a new text handler, more code changes are needed.

If you want to implement them and make PRs, I'm happy too.

[ashb]

Add a space after ':' ',', I though you want this

{"logger": "test", "time": "abc", "http": {"other": "value"}}

Oops yes, that is what I meant

[icefed]

Features SpaceIndent and LevelStringer has been added, you can have a try.
@ashb