Add vue/one-component-per-file rule. (#671)

Author: armano2

docs/rules/one-component-per-file.md

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+# enforce that each component should be in its own file
+
+## :book: Rule Details
+
+This rule checks if there is oly one component per file.
+
+:-1: Examples of **incorrect** code for this rule:
+
+```js
+Vue.component('TodoList', {
+  // ...
+})
+
+Vue.component('TodoItem', {
+  // ...
+})
+```
+
+:+1: Examples of **correct** code for this rule:
+
+```js
+export default {
+  name: 'my-component'
+}
+```
+
+## :wrench: Options
+
+Nothing.
+
+## :books: Further reading
+
+- [Style guide - Component files](https://vuejs.org/v2/style-guide/#Component-files-strongly-recommended)

lib/rules/one-component-per-file.js

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+/**
+ * @fileoverview enforce that each component should be in its own file
+ * @author Armano
+ */
+'use strict'
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'enforce that each component should be in its own file',
+      category: undefined, // strongly-recommended
+      url: 'https://github.com/vuejs/eslint-plugin-vue/blob/v5.0.0-beta.5/docs/rules/one-component-per-file.md'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      toManyComponents: 'There is more than one component in this file.'
+    }
+  },
+  create (context) {
+    let componentCount = 0
+
+    return Object.assign({},
+      utils.executeOnVueComponent(context, () => {
+        ++componentCount
+      }),
+      {
+        'Program:exit' (node) {
+          if (componentCount > 1) {
+            context.report({
+              node: node,
+              messageId: 'toManyComponents'
+            })
+          }
+        }
+      }
+    )
+  }
+}

tests/lib/rules/one-component-per-file.js

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+/**
+ * @fileoverview enforce that each component should be in its own file
+ * @author Armano
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/one-component-per-file')
+const RuleTester = require('eslint').RuleTester
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2018,
+    sourceType: 'module'
+  }
+})
+
+ruleTester.run('one-component-per-file', rule, {
+  valid: [
+    {
+      filename: 'test.js',
+      code: `Vue.component('name', {})`
+    },
+    {
+      filename: 'test.js',
+      code: `
+        Vue.component('name', {})
+        new Vue({})
+      `
+    },
+    {
+      filename: 'test.js',
+      code: `
+        const foo = {}
+        new Vue({})
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `export default {}`
+    },
+    {
+      filename: 'test.vue',
+      code: `export default {
+        components: {
+          test: {
+            name: 'foo'
+          }
+        }
+      }`
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.js',
+      code: `
+        Vue.component('name', {})
+        Vue.component('name', {})
+      `,
+      errors: [{
+        message: 'There is more than one component in this file.'
+      }]
+    },
+    {
+      filename: 'test.js',
+      code: `
+        Vue.component('TodoList', {
+          // ...
+        })
+        
+        Vue.component('TodoItem', {
+          // ...
+        })
+        export default {}
+      `,
+      errors: [{
+        message: 'There is more than one component in this file.'
+      }]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        Vue.component('name', {})
+        export default {}
+      `,
+      errors: [{
+        message: 'There is more than one component in this file.'
+      }]
+    }
+  ]
+})