fix(language-plugin-pug): pre-map HTML to Pug offset attribute

[johnsoncodehk]

Fix #5899

The issue was introduced by the post-processing of normalizeTemplateAST in #5782.

When normalizeTemplateAST calls createIfBranch, it retrieves the offset to calculate the position of the conditional expression. Since @vue/language-plugin proxies the offset property, createIfBranch obtains the offset based on the Pug position.

Subsequently, the node with the Pug position is added to the AST tree. The proxy from @vue/language-plugin establishes a proxy for the new node, causing the next retrieval of offset to map the already Pug-positioned offset from HTML to Pug again, resulting in an incorrect outcome.

The issue occurs in normalizeIfBranch, so it only happens when a Pug template uses both v-slot and v-if on the same node.

The solution is to avoid using a proxy and perform the HTML to Pug conversion in advance within compileSFCTemplate.