Just concatenating the strings is much faster.
Before:
```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8 8194791 139 ns/op
BenchmarkAttr/name-value_attributes-8 5143292 229 ns/op
PASS
ok github.com/maragudk/gomponents 2.841s
```
After:
```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8 16755404 67.0 ns/op
BenchmarkAttr/name-value_attributes-8 10208625 116 ns/op
PASS
ok github.com/maragudk/gomponents 2.702s
```
