fix(plugin-dehydrate): find string in a right way

[oott123]

Summary

Which package does this PR involve? (check one)

• [ ] root project
• [ ] vuepress-plugin-clean-urls
• [ ] vuepress-plugin-container
• [x] vuepress-plugin-dehydrate
• [ ] vuepress-plugin-medium-zoom
• [ ] vuepress-plugin-named-chunks
• [ ] vuepress-plugin-nprogress
• [ ] vuepress-plugin-redirect
• [ ] vuepress-plugin-smooth-scroll
• [ ] vuepress-plugin-table-of-contents
• [ ] vuepress-plugin-typescript
• [ ] vuepress-plugin-zooming
• [ ] vuepress-mergeable
• [ ] vuepress-types

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

• [x] Bugfix (fix)
• [ ] Feature (feat)
• [ ] Performance enhancement (perf)
• [ ] Code Style (style)
• [ ] Refactor (refactor)
• [ ] Docs (docs)
• [ ] Build-related changes (build)
• [ ] CI-related changes (ci)
• [ ] Testing (test)
• [ ] Other, please describe:

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

• [ ] Yes
• [x] No

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

The PR fulfills these requirements:

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

Other information:

String.prototype.find will treat the first argument as regexp.

The vairable resourceOriginal, which contents is {{{ renderResourceHints() }}}, was not works as expected because () was treated as regex match groups. This leads to many unused tags generated in the ssr template.

indexOf is safe for any string to find the neddle in string.

[meteorlxy]

Why do you think current usage is not correct?

[oott123]

@meteorlxy because it wont find renderResourceHints tag correctly and it will add '<!--before-resource-hints-->' and '<!--after-resource-hints-->' tags every time you build the site, no matter they exists or not.

[meteorlxy]

@oott123 Oh nice catch. Thanks :heart:

cc @Shigma