feat($theme-default): $mobileSidebarWidth

[Mister-Hope]

allow modifying $mobileSidebarWidth

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

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• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[billyyyyy3320]

It could be useful 👍.

However, if it's gonna be a overridable variable, imo it should be moved to default preset and mentioned in corresponding section.

[Mister-Hope]

This feature will be solved in #2301. @newsbielt703 And I would be glad if you can spend some time checking that PR.

I moved some variables to @vuepress/core in that PR.