tpl/tplimpl: Fix template truth logic

Before this commit, due to a bug in Go's `text/template` package, this would print different output for typed nil interface values:

```
{{ if .AuthenticatedUser }}User is authenticated!{{ else }}{{ end }}
{{ if not .AuthenticatedUser }}{{ else }}}User is authenticated!{{ end }}
```

This commit works around this by wrapping every `if` and `with` with a custom `getif` template func with truth logic that matches `not`, `and` and `or`.

Those 3 template funcs from Go's stdlib are now pulled into Hugo's source tree and adjusted to support custom zero values, e.g. types that implement `IsZero`.

This means that you can now do:

```
{{ with .Date }}{{ . }}{{ end }}
```

And it would work as expected.

Fixes #5738

common/types/types.go

@@ -50,3 +50,9 @@ func NewKeyValuesStrings(key string, values ...string) KeyValues {
 	}
 	return KeyValues{Key: key, Values: iv}
 }
+
+// Zeroer, as implemented by time.Time, will be used by the truth template
+// funcs in Hugo (if, with, not, and, or).
+type Zeroer interface {
+	IsZero() bool
+}