fix(xmind2xmindmark): fix indentation error when converting xmind to xmindmark

[jinzcdev]

Issue Description

According to the xmindmark syntax documentation, the correct format for .xmindmark text should be:

Central Topic
- Main Topic 1
- Main Topic 2

I use the command xmindmark -f xmind example.xmindmark to generate the example.xmind file, which is a valid .xmind file.

---

However, when I use the command xmindmark from example.xmind, the generated example.xmindmark content has the following format:

$ xmindmark from example.xmind
Central Topic
    - Main Topic 1
    - Main Topic 2

When I attempt to convert example.xmindmark back to example.xmind using the xmindmark command, a parsing error occurs:

$ xmindmark -f xmind example.xmindmark
/Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/parser/mindmap.js:229
    parentObject = parentObject.rootTopic || parentObject;
                                ^

TypeError: Cannot read properties of undefined (reading 'rootTopic')
    at addTopic (/Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/parser/mindmap.js:229:33)
    at addLine (/Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/parser/mindmark.js:69:65)
    at /Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/parser/mindmark.js:146:62
    at Array.forEach (<anonymous>)
    at createMapByXMindMark (/Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/parser/mindmark.js:146:46)
    at /Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/lib/xmindmark-to-xmind.js:34:64
    at Generator.next (<anonymous>)
    at /Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/lib/xmindmark-to-xmind.js:8:71
    at new Promise (<anonymous>)
    at __awaiter (/Users/{me}/.nvm/versions/node/v22.11.0/lib/node_modules/xmindmark/dist/src/lib/xmindmark-to-xmind.js:4:12)

Root Cause: The indentation handling in src/lib/xmind-to-xmindmark.ts is incorrect, causing the generated example.xmindmark file to deviate from the standard xmindmark syntax. Additionally, the original unit tests were also flawed.

Solution

Modify the indentation logic in src/lib/xmind-to-xmindmark.ts.