[BUG] Code Block Display and Diff Generation Issue with /edit Command in Markdown Documents

Before submitting your bug report

[X] I believe this is a bug. I'll try to join the Continue Discord for questions
[X] I'm not able to find an open issue that reports the same bug
[X] I've seen the troubleshooting guide on the Continue Docs

Relevant environment info

- Continue: v0.9.4

Description

When a user writes a code block in a markdown document and later uses the /edit slash command on this code block, the code block isn't accurately displayed in the code preview and is entirely eliminated from the resulting diff.

Possible solution idea?

The display problem of the code block within your preview might be resolved for most cases by using an increased number of backticks.

In markdown, typically a single backtick starts inline code and three backticks start a code block. However, you can use any number of backticks, just ensure the number of backticks used to start and end a code block is the same.

For example, this code block starts and ends with nine backticks, and everything inside is properly displayed. If I had used only eight backticks, the last code block would not render properly in markdown.

This is a code block with 3 backticks
```js
const factorialize = x => x < 1 ? 1 : factorialize(x - 1) * x;
factorialize(5); // 120

This is a code block with 8 backticks

const even = n => n >= 0 ? (!(n % 2) && console.log(n, 'is even'), even(n - 1)) : null;
even(3);
// 2 is even
// 0 is even


The change below ensures that a code block, marked by three backticks, is not removed from the diff.
However, more changes are required to fully resolve this issue. These include updating the
documentation and templates, and fixing the preview code when highlighted in the 'continue'
extension. 

```diff
--- a/core/commands/slash/edit.ts
+++ b/core/commands/slash/edit.ts
@@ -188,7 +188,7 @@ function isEndLine(line: string) {

 function lineToBeIgnored(line: string, isFirstLine: boolean = false): boolean {
   return (
-    line.includes("```") ||
+    line.includes("````") ||
     line.includes("<modified_code_to_edit>") ||
     line.includes("<file_prefix>") ||
     line.includes("</file_prefix>") ||

The idea of using extra backticks in markdown for properly formatting inline code or code blocks was obtained from a video[^1] produced by a VSCode maintainer who also developed numerous VSCode markdown extensions[^2].

[^1]: YouTube: Markdown Tips — Using backticks in inline code [^2]: Publisher Matt Bierner - Visual Studio Marketplace

To reproduce

Create a markdown file that includes some text and a code block as shown below:

To print Abbreviated weekday name:
```bash
date +"%a"



~~2. Highlight the code block you've written and use the `/edit` command to modify it. Note that the preview will not display your code block correctly.~~ 

==> **Addressed in #742**

<img src="https://github.com/continuedev/continue/assets/92653266/aee2d7e1-635b-4421-800a-e53558fd8267" width="400">

3. Observe that the diff generated no longer includes the lines that initiate and end the code block.

<img src="https://github.com/continuedev/continue/assets/92653266/3941dce8-7dce-4ff4-8cdb-48d9726dde09" width="800">

### Log output

_No response_

This change correctly displays a codeblock with three backticks in the preview. However, the same issue may arise if a user uses four backticks.

--- a/gui/src/components/markdown/CodeSnippetPreview.tsx
+++ b/gui/src/components/markdown/CodeSnippetPreview.tsx
@@ -138,9 +138,9 @@ function CodeSnippetPreview(props: CodeSnippetPreviewProps) {
       </PreviewMarkdownHeader>
       <pre className="m-0" ref={codeBlockRef}>
         <StyledMarkdownPreview
-          source={`\`\`\`${getMarkdownLanguageTagForFile(
+          source={`\`\`\`\`${getMarkdownLanguageTagForFile(
             props.item.description
-          )}\n${props.item.content}\n\`\`\``}
+          )}\n${props.item.content}\n\`\`\`\``}
           maxHeight={MAX_PREVIEW_HEIGHT}
           showCodeBorder={false}
         />

@sestinj, could the solution be to find the maximum number of consecutive backticks used and then use one more backtick?

continuedev / continue