Block edit renderer

[beyang]

Render a proposed edit as a find-replace block:

Adds a test command under the shortcut ctrl + Enter. This test command parses a proposed edit from the last instance in the current file of a string like this:

<<<<
String to replace
====
Replacement string
~~~~
{ "minReplacerCol": 80 }
>>>>

It then applies this edit in the file.

The test command only serves to show the format of the renderer. The rendering function is renderBlockEdit. This is what should be reused to displaying proposed edits.

To try out the test command, open up a file like ContextCell.tsx (or any file with occurrences of the find-replace pattern above) and run the ctrl + Enter shortcut at a location after the find-replace pattern.