refactor: replace logger

[dmke]

This replaces the zap logging engine with Go 1.21's new structured logger, log/slog, or more precisely a thin wrapper around that (xlog).

The log/slog package has a few things missing, which are present in xlog:

1. xlog provides a no-op logger, which simply discards any log output. This is extensively used in our tests.
2. xlog has a Fatal() output method, which simply calls Error() and then os.Exit(1).
3. xlog does not provide global convenience functions to access a default logger instance; all calls must go through a Logger instance created with New().
4. xlog treats error values as first-class citizen. Since (log/slog).Error() is a convenience function for their default logger instance, there is no built-in way to treat errors as values. In comparison, (xlog).Error(err) constructs an slog.Attr.

Point (2) is debatable, since xlog.Fatal is only used in cmd/texd/main.go, so I'd be willing to forfeit it.

Some TODOs remain:

• [ ] xlog, i.e. its extension over log/slog, is not really tested
• [x] more documentation
• [x] switch to functional options for xlog.New()
• [ ] colorization (at least for local development)

Issue: #1

[codecov[bot]]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (5712300) 80.92% compared to head (4ce3741) 80.92%.

Files	Patch %	Lines
service/renderer.go	44.44%	10 Missing :warning:
service/service.go	57.14%	3 Missing :warning:
service/middleware/logging.go	88.88%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #108 +/- ## ======================================= Coverage 80.92% 80.92% ======================================= Files 30 30 Lines 1594 1594 ======================================= Hits 1290 1290 Misses 243 243 Partials 61 61 ```

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