feat(legacy): add 'assumptions' option

[xiongmao86]

Description

Babel allows specifying compiler assumptions for generating small output. This pr adds assumptions option to pass to babel transform inside @vitejs/plugin-legacy.

Fixes #6965

Additional context

none

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[xiongmao86]

When I am trying to write a test, I meet the following questions:

• Should it reside in playground/vue-legacy?
• What input code should I choose?
• Where should I put it so that it will show up in JavaScript code after typescript transpiling?

Can anyone please give me some pointers. Thanks.

[xiongmao86]

@babel/core and @babel/stand-alone version is newer than Babel 7.13.0, Why the ci test warn that assumptions is not supported?