feat: add logger package and tests

[mkcp]

Description

This PR adds a logger package using log/slog and tests. It's extracted from the proof of concept in #3101.

Related Issue

Relates to #2576

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed

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[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pkg/logger/logger.go	95.65%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/pkg/logger/logger.go	95.65% <95.65%> (ø)

[mkcp]

I reintroduced the package level default logger here rather than passing into slog's default, and added more clarity on how it's intended to be used as a development aid.

When adding a Default().Info() call to init(),

we get a usable default logger:

[schristoff]

beautiful