feat(GeneratorAPI): accept multiple arguments for the resolve method (vuejs#4715)

* feat(GeneratorAPI): accept multiple arguments for the resolve method

To make its signature conform to  the standard `path.resolve`, avoid
confusions.

There's already a bug in the downstream plugin that was caused by such
confusion: https://github.com/Akryum/vue-cli-plugin-ssr/blob/a742953b8376817950a01ad79967b9aa02da5388/generator/index.js#L163-L171

* docs: update descriptions of the `resolve` api [ci skip]

Author: haoqunjiang

docs/dev-guide/generator-api.md

@@ -44,10 +44,10 @@ The version string for the **project local** `@vue/cli-service` version that is
 ## resolve
 
 - **Arguments**
-  - `{string} _path` - relative path from project root
+  - `{string} ..._paths` - A sequence of relative paths or path segments
 
 - **Returns**
-  - `{string}`- the resolved absolute path
+  - `{string}`- the resolved absolute path, caculated based on the current project root
 
 - **Usage**:
 Resolve a path for the current project

packages/@vue/cli/lib/GeneratorAPI.js

@@ -68,11 +68,11 @@ class GeneratorAPI {
   /**
    * Resolve path for a project.
    *
-   * @param {string} _path - Relative path from project root
-   * @return {string} The resolved absolute path.
+   * @param {string} _paths - A sequence of relative paths or path segments
+   * @return {string} The resolved absolute path, caculated based on the current project root.
    */
-  resolve (_path) {
-    return path.resolve(this.generator.context, _path)
+  resolve (..._paths) {
+    return path.resolve(this.generator.context, ..._paths)
   }
 
   get cliVersion () {