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

tpl/compare/init.go

@@ -71,6 +71,27 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.And,
+			[]string{"and"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.Or,
+			[]string{"or"},
+			[][2]string{},
+		)
+
+		// getif is used internally by Hugo. Do not document.
+		ns.AddMethodMapping(ctx.getIf,
+			[]string{"getif"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.Not,
+			[]string{"not"},
+			[][2]string{},
+		)
+
 		ns.AddMethodMapping(ctx.Conditional,
 			[]string{"cond"},
 			[][2]string{