fix(core): correct offset calculation in mergeWhitespaceTokens (#1162)

Author: Karthikeya1500

packages/core/src/highlight/code-to-hast.ts

@@ -287,15 +287,15 @@ function mergeWhitespaceTokens(tokens: ThemedToken[][]): ThemedToken[][] {
   return tokens.map((line) => {
     const newLine: ThemedToken[] = []
     let carryOnContent = ''
-    let firstOffset = 0
+    let firstOffset: number | undefined
     line.forEach((token, idx) => {
       const isDecorated = token.fontStyle && (
         (token.fontStyle & FontStyle.Underline)
         || (token.fontStyle & FontStyle.Strikethrough)
       )
       const couldMerge = !isDecorated
       if (couldMerge && token.content.match(/^\s+$/) && line[idx + 1]) {
-        if (!firstOffset)
+        if (firstOffset === undefined)
           firstOffset = token.offset
         carryOnContent += token.content
       }
@@ -304,20 +304,20 @@ function mergeWhitespaceTokens(tokens: ThemedToken[][]): ThemedToken[][] {
           if (couldMerge) {
             newLine.push({
               ...token,
-              offset: firstOffset,
+              offset: firstOffset!,
               content: carryOnContent + token.content,
             })
           }
           else {
             newLine.push(
               {
                 content: carryOnContent,
-                offset: firstOffset,
+                offset: firstOffset!,
               },
               token,
             )
           }
-          firstOffset = 0
+          firstOffset = undefined
           carryOnContent = ''
         }
         else {