Delete per-phase dirs from /devtools-extension/

[mikehoffms]

This PR is ready to merge, but don't merge this PR until merge PR 3616.

Related PRs:

• Convert "Create a DevTools extension" to sample-based MicrosoftDocs/edge-developer#3616 - Docs repo
• Delete per-phase dirs from /devtools-extension/ #106 - Demos repo

Branch dir:

• https://github.com/mikehoffms/Demos/tree/user/mikehoffms/devtools-extension/devtools-extension

Changes:

• Merged code from phase 3 dir (Memory API) & phase 4 dir (2-way messaging).
• Moved code up from subdirs directly into a single dir.

AB#59925391

[mikehoffms]

LGTM - Writer/Editor