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/jsctx_string.go

@@ -0,0 +1,16 @@
+// Code generated by "stringer -type jsCtx"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _jsCtx_name = "jsCtxRegexpjsCtxDivOpjsCtxUnknown"
+
+var _jsCtx_index = [...]uint8{0, 11, 21, 33}
+
+func (i jsCtx) String() string {
+	if i >= jsCtx(len(_jsCtx_index)-1) {
+		return "jsCtx(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _jsCtx_name[_jsCtx_index[i]:_jsCtx_index[i+1]]
+}