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/attr_string.go

@@ -0,0 +1,16 @@
+// Code generated by "stringer -type attr"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _attr_name = "attrNoneattrScriptattrScriptTypeattrStyleattrURLattrSrcset"
+
+var _attr_index = [...]uint8{0, 8, 18, 32, 41, 48, 58}
+
+func (i attr) String() string {
+	if i >= attr(len(_attr_index)-1) {
+		return "attr(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _attr_name[_attr_index[i]:_attr_index[i+1]]
+}