tpl: Put Go's internal template funcs in Hugo's map

tpl: Put Go's internal template funcs in Hugo's map

```
name                            old time/op    new time/op    delta
SiteNew/Many_HTML_templates-16    43.4ms ± 0%    42.7ms ± 0%  -1.71%  (p=0.016 n=4+5)

name                            old alloc/op   new alloc/op   delta
SiteNew/Many_HTML_templates-16    17.5MB ± 0%    17.5MB ± 0%    ~     (p=0.690 n=5+5)

name                            old allocs/op  new allocs/op  delta
SiteNew/Many_HTML_templates-16      247k ± 0%      247k ± 0%    ~     (p=0.310 n=5+5)
```

Fixes #6717