chore: Reduce conversion processes

[thompson-tomo]

Progresses #153
Progresses #258
Progresses #279

This works to further improve the code path and reduce un-necessary operations during the file handling process.

Previously we had:

• File content was being read
• content was being split based on EOL
• content was being parsed line by line looking for toc
• file content was transformed to ast to get markdown headers
• file content was transformed to ast to get html headers
• toc string built
• toc split into lines
• toc lines spliced into content lines
• content lines merged back into content

Now we just have

• File content was being read
• file content was transformed to ast
• get toc from ast
• get markdown headers from ast
• get html headers from ast
• toc string built
• content built by merging content to prior old toc, new toc & after old toc.

Hence with this new approach the transformation is reduced & ensures that we only touch the toc and not the entire doc.

This will hopefully improve performance of larger files.