Introduce runtime_toolchain_type

According to discussions in #3854 having two toolchains of the same type for different things is troublesome. It's better to have separate runtime as well as compile toolchains.

This commit creates a new runtime_toolchain_type and registers toolchains without execution constraints for this type. Once merged, rules_ts can start consuming the new toolchain type in its js_binary rule to ensure the correct Node for the correct target environment is selected.

Fixed [Bug]: Execution toolchain defined without `target_compatible_with` makes it a candidate to selection

Fixes #3854
Work towards #3795

Author: guw

nodejs/node_toolchain_alias.bzl

@@ -53,7 +53,7 @@ node_host_runtime_alias = rule(
     implementation = _node_host_runtime_alias,
     attrs = {
         "_runtime": attr.label(
-            default = Label("//toolchains:current_node_runtime"),
+            default = Label("//nodejs:current_node_runtime"),
             providers = [
                 NodeInfo,
                 platform_common.TemplateVariableInfo,