fix: set edit context to undefined in dispose

[SimonSiefke]

This fixes a memory leak related to edit context.

When opening and closing a text editor, the number of EditContexts seems to increase each time. Setting the editContext property to undefined in the dispose function seems to solve this issue.

Test Script

git clone git@github.com:SimonSiefke/vscode-memory-leak-finder.git &&
cd vscode-memory-leak-finder &&
git checkout v5.88.0 &&
npm ci &&
node packages/cli/bin/test.js --cwd packages/e2e  --check-leaks --measure-after --measure edit-context-count --only ^editor.open --runs 97 &&
cat ./.vscode-memory-leak-finder-results/edit-context-count/editor.open.json 

Before

The number of edit contexts when opening and closing a text editor 97 times grows from 2 to 99:

After

The number of edit contexts when opening and closing a text editor 97 times stays constant:

[ivanstepanovftw]

Hello! Please, review and merge #154465