feat(plugin-legacy): support file protocol

[wmz46]

Description

Sometimes I want the packaged web page to run locally without using services, such as "file:///C:/demo/index.html".
In chrome, CORS errors may occur when using local scripts for module scripts.
So I need to use '@vitejs/plugin-legacy' to package it as a nomodule script.
At this time, I need to be able to inject and run nomodule scripts by distinguishing web protocol.

tr. 有时候我希望打包之后的网页能在本地运行，不使用服务，如"file:///C:/demo/index.html"。
在chrome由于module scripts使用本地脚本会出现跨域的错误。
所以我需要使用'@vitejs/plugin-legacy'将其打包为nomodule的脚本。
这时我需要能通过区分网页协议来注入和运行nomodule脚本。

fixes #12841
closes #2574

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

I think it's fine to have this. This is more simple than #2574. This adds a little code size for uses that doesn't worry about file protocol. But I think it's ok as the size is very small.