fix(plugin-vue): add newline character before class components

[ygj6]

Description

fix #2787

This issue is caused by 2900a9a.

var str =  `import { Vue, Options } from "vue-class-component";

export default class ClassComponent extends Vue {
  counter = 0
}`;

After replace, it will become：

import { Vue, Options } from "vue-class-component"class ClassComponent extends Vue {
  counter = 0
}

At first I plan to optimize exportDefaultClassRE expression:

-const exportDefaultClassRE = /(?:(?:^|\n|;)\s*)export\s+default\s+class\s+([\w$]+)/
+const exportDefaultClassRE = /(^|\n|;)\s*export\s+default\s+class\s+([\w$]+)/ 

-script.content.replace(exportDefaultClassRE, `class $1`)
+script.content.replace(exportDefaultClassRE, `$1 class $2`)

the above solution could solve both #2787 and #2277, but I am worried that there are other cases that have not been considered, so I think it is better to add \n before class:

scriptCode = script.content.replace(exportDefaultClassRE, `\nclass $1`)

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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