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/urlpart_string.go

@@ -0,0 +1,16 @@
+// Code generated by "stringer -type urlPart"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _urlPart_name = "urlPartNoneurlPartPreQueryurlPartQueryOrFragurlPartUnknown"
+
+var _urlPart_index = [...]uint8{0, 11, 26, 44, 58}
+
+func (i urlPart) String() string {
+	if i >= urlPart(len(_urlPart_index)-1) {
+		return "urlPart(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _urlPart_name[_urlPart_index[i]:_urlPart_index[i+1]]
+}