Create lightweight forks of text/template and html/template

This commit also removes support for Ace and Amber templates.

Updates #6594

tpl/internal/go_templates/htmltemplate/element_string.go

@@ -0,0 +1,16 @@
+// Code generated by "stringer -type element"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _element_name = "elementNoneelementScriptelementStyleelementTextareaelementTitle"
+
+var _element_index = [...]uint8{0, 11, 24, 36, 51, 63}
+
+func (i element) String() string {
+	if i >= element(len(_element_index)-1) {
+		return "element(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _element_name[_element_index[i]:_element_index[i+1]]
+}