[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>

test/envparsing.sh

@@ -137,6 +137,7 @@ EXPECT=$(cat << "EOF"
     "local-only": false,
     "log-client-ip": false,
     "log-db-queries": true,
+    "log-format": "json",
     "log-level": "info",
     "log-timestamp-format": "banana",
     "media-cleanup-every": 86400000000000,
@@ -223,6 +224,7 @@ OUTPUT=$(GTS_LOG_LEVEL='info' \
 GTS_LOG_TIMESTAMP_FORMAT="banana" \
 GTS_LOG_DB_QUERIES=true \
 GTS_LOG_CLIENT_IP=false \
+GTS_LOG_FORMAT=json \
 GTS_APPLICATION_NAME=gts \
 GTS_LANDING_PAGE_USER=admin \
 GTS_HOST=example.com \