switch to using vscode.workspace.fs for loading keplergl maps from remote workspaces

[RandomFractals]

should also work with new github/codespaces beta, in addition to keplergl configs already served & loaded via https.

see: microsoft/vscode#104199 (comment)

• https://code.visualstudio.com/updates/v1_37?ref=codebldr#_vscodeworkspacefs

• https://code.visualstudio.com/api/advanced-topics/remote-extensions#incorrect-execution-location

[RandomFractals]

looks just fine in the new github codespaces beta ...