feat(preset-mini)!: allow empty string for variablePrefix

[chu121su12]

Close #2085

Tagged as breaking since previously empty string will become the default --un instead.

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[zyyv]

How about set the type of variablePrefix to boolean | string?

• If variablePrefix's value is false, it means to disable variablePrefix default string(use empty string).
• If variablePrefix's value is true, it means use un-.
• If it's a string, it means user custom variablePrefix.