Test merge conflict editor

[aeschli]

Testing #27150

Complexity 2

• Linux @aeschli
• Windows @michelkaporin
• Mac @isidorn

From #27288 (comment):
To simulate a merge conflict, create a file with the following content

aaa
bbb
<<<<<<< HEAD
ccc
ccc
=======
ddd
ddd
>>>>>>> somebranch
eee
eee

• Test the new 'merge' commands in the command palate
• Test the code lens actions (set "editor.codeLens": true)
• Test that you can theme the merge editor with these colors:
  merge.currentContentBackground
merge.currentHeaderBackground
merge.incomingContentBackground
merge.incomingHeaderBackground
overviewRuler.currentContentForeground
overviewRuler.incomingContentForeground