[feature + performance] add JSON logging format (#4355)

# Description

Adds JSON logging as an optional alternative log output format. In the process this moves our log formatting itself into a separate subpkg to make it more easily modular, and improves caller name getting with some calling function name caching.

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4355
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

vendor/modules.txt

@@ -232,6 +232,9 @@ codeberg.org/gruf/go-byteutil
 codeberg.org/gruf/go-cache/v3
 codeberg.org/gruf/go-cache/v3/simple
 codeberg.org/gruf/go-cache/v3/ttl
+# codeberg.org/gruf/go-caller v0.0.0-20250806133437-db8d0b1f71cf
+## explicit; go 1.24.5
+codeberg.org/gruf/go-caller
 # codeberg.org/gruf/go-debug v1.3.0
 ## explicit; go 1.16
 codeberg.org/gruf/go-debug
@@ -255,7 +258,7 @@ codeberg.org/gruf/go-iotools
 ## explicit; go 1.20
 codeberg.org/gruf/go-kv
 codeberg.org/gruf/go-kv/format
-# codeberg.org/gruf/go-kv/v2 v2.0.3
+# codeberg.org/gruf/go-kv/v2 v2.0.5
 ## explicit; go 1.24
 codeberg.org/gruf/go-kv/v2
 codeberg.org/gruf/go-kv/v2/format