decoder: add support for nested log formatting

[andresovela]

This PR is a follow-up for #769.

This adds support for nesting formats, allowing users to specify formatting for groups of log segments at once.
For example, "{[{L:bold}]%underline} {s}" prints "[ERROR] Something went wrong" where ERROR is bold, and [ERROR] is underlined from [to ].

This nesting feature also enables aligning groups of log segments at once, which allows users to format their logs like this:

   54251 [INFO ] main.rs:543   Something happened
  400241 [ERROR] parser.rs:86  Failed to parse input 

I think the formatting feature is pretty much complete with this PR, and I don't have plans to extend it any further at this point.

That said, I do intend to submit a PR to propose a new default format after this is merged.

I'm looking forward to finally have this in defmt :)

[Urhengulas]

Thank you. I have two questions, but otherwise it seems good to me.