We still have go-toml as a transitive dependency, and it is the way to go eventually, but we care about speed, so let us wait that one out.
Note that the issue this fixes is about taxonomies, but I guess this is a general issue for sites with many pages that uses TOML as front matter.
```
benchmark old ns/op new ns/op delta
BenchmarkFrontmatterTags/TOML:1-4 23206 8543 -63.19%
BenchmarkFrontmatterTags/TOML:11-4 80117 18495 -76.92%
BenchmarkFrontmatterTags/TOML:21-4 140676 28727 -79.58%
benchmark old allocs new allocs delta
BenchmarkFrontmatterTags/TOML:1-4 173 60 -65.32%
BenchmarkFrontmatterTags/TOML:11-4 625 138 -77.92%
BenchmarkFrontmatterTags/TOML:21-4 1106 210 -81.01%
benchmark old bytes new bytes delta
BenchmarkFrontmatterTags/TOML:1-4 9231 2912 -68.45%
BenchmarkFrontmatterTags/TOML:11-4 19808 5184 -73.83%
BenchmarkFrontmatterTags/TOML:21-4 31200 7536 -75.85%
```
See #3541
Updates #3464
