addfindmatchtoselection in notebooks has different behavior for single characters

[amunger]

a couple cases that mis-behave

1. open CITestResults.ipynb in vscode-jupyter repo
2. in the configuration section, in the code with open('.github_token', 'r') as f: highlight just the f at the end
3. press ctrl+d several times

🐛 loops around within that cell

1. same file, same cell, highlight f. on the second line
2. press ctrl+d several times

🐛 selection changes to just f and goes through and highlights f characters from all cells

Originally posted by @amunger in #235117