feat($shared-utils): remove leading slash of regularPath when getting permalink

[billyyyyy3320]

Summary

relevant to #1783

:regular is different with all the other template variables(only :regular can't go before a slash).

Besides, I found VuePress can't work fine if someone config like below(although It‘s a meaningless config), and this PR will also solve this problem:

module.exports = {
  permalink: ":regular"
};

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
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Does this PR introduce a breaking change? (check one)

• Yes
• No

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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)

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• Related documents have been updated
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Other information:

[vue-bot]

Hey @newsbielt703, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚