Add Dev Container

[michielbdejong]

Adding the .devcontainer.json file is harmless to people who don't use it.
I added a note about its availability in the readme.
And I made the following changes

• add scripts/dev to wrap around the initial install & reset commands (again, the presence of this file should be harmless to people who don't use it)
• Change export NVM_DIR="$HOME/.nvm" to export NVM_DIR="${NVM_DIR:-$HOME/.nvm}" everywhere, to make this oneliner robust against environments (such as this one) that use a different NVM_DIR than ~/.nvm.
• add an nvm install command to scripts/add, so that for instance node-solid-server can get its own preferred node version. As far as I know this command is a noop if the requested version from .nvmrc is already available on the host system (but if not then we need to think this through some more).

[michielbdejong]

I added the scripts/watch-css to scripts/dev and that now takes like 15 minutes.

Also, there is a build error:

[source-pane] [solid-panes] /workspaces/solidos/workspaces/solid-panes/src/versionInfo.ts
[source-pane] [solid-panes]   6:2  error  Parsing error: Invalid character

And although I can use CSS+mashlib on http://localhost:3000 as desired, I'm apparently not able to edit solid-panes code and then see the changes reflected. Maybe I should try building mashlib & restarting css manually

[timea-solid]

Thank you for the improvements.