preference editor take up a lot of memory

[fanyipin]

Bug Description:

When checking the memory of browserwindow, I find that markdownit will occupy a lot of memory over 100M in preference editor, mainly because linkify has a large number of instances, and each instance corresponds to many regular expressions as shown in the figure above. I wonder if this can be optimized:

1. Check whether a single markdownit instance can meet the requirements
2. Whether multiple markdownit instances can share a linkify regular expression set
3. Can markdownRenderer be destroyed after preference editor rendering is completed

Additional Information

• Operating System:
• Theia Version: 1.39