Allow building with Zig 0.15

[agagniere]

• In build.zig, specify root_module
• Bump dependency for 0.15
• Use the new {f} formatter syntax
• Adapt IOs post writergate

[agagniere]

Depends on rockorager/zeit#39

[agagniere]

The first commit is still compatible with zig 0.14
The second breaks the API so I bumped the version

[agagniere]

To review:

• I removed the buffering in the config, because now all writers have a buffer (that can be of size 0), so the choice to bufferize or not is made outside of Axe.
• I added a flush_each_log in the config to choose if the writers are flushed after each log.
• Regardless of the config, stderr is buffered and flushed after each log. It might make more sense to take config into account here, which would involve adding a buffer for stderr in the axe struct

[Ratakor]

Great work!

[agagniere]

OK, added a test for strftime, only checking the number of characters written, so now the relevant code is actually compiled.

[agagniere]

Wait I forgot to add a way to flush all writers at the end

[agagniere]

OK, should be good now !
I added a public flush function, called in deinit, for users to be able to force flush any time

[Ratakor]

Thank you!