build($theme-default): bump stylus version

[JoeyBling]

Use new version to support @extends writing

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:

fix #2227

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

• Chrome
• Firefox
• Safari
• Edge
• 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]

I don't think such feature will be added in a patch version. Then, I noticed that Stylus has supportted @extends since 0.22.0 by this commit.

[billyyyyy3320]

Plus, I cannot reproduce the issue you faced. Could you create another issue with a reproduce approach?

[JoeyBling]

@billyyyyy3320 Steps to reproduce：

1. Create a test folder
2. Install the last version of 1.5.3 npm install vuepress@1.5.3 -S
3. Install default stylus version 0.54.5 npm install stylus@0.54.5 -S
4. Install the latest version of vuepress npm install vuepress@latest -S
5. Launch vuepress dev

Console error:

Reason: Error: Module build failed (from ./node_modules/stylus-loader/index.js):
Error: C:\Users\24343\Desktop\vuepress-test\node_modules\@vuepress\theme-default\components\DropdownLink.vue:156:15
   152|       vertical-align middle
   153|       margin-top -1px
   154|       margin-left 0.4rem
   155|   .mobile-dropdown-title
   156|     @extends .dropdown-title
----------------------^
   157|     display none
   158|     font-weight 600
   159|     font-size inherit

Failed to @extend ".dropdown-title"

[meteorlxy]

This should be a "build" commit instead of a "fix" commit.

I think it's OK to bump stylus to a newer patch version