Categorise things like SDK paths as machine-specific

[DanTup]

https://code.visualstudio.com/updates/v1_34#_machinespecific-settings

Otherwise we'll get weird issues running on containers because local paths are passed in:

ERR EINVAL: invalid argument, open '/workspaces/vscode-dev-containers/containers/flutter/M:\Coding\Applications\Dart-Code-Logs\insiders.analyzer.txt': Error: EINVAL: invalid argument, open '/workspaces/vscode-dev-containers/containers/flutter/M:\Coding\Applications\Dart-Code-Logs\insiders.analyzer.txt'

[DanTup]

This is tricky...

Machine-specific settings can only be configured for User settings.

Many of our paths are "window" scoped, so that they can be defined within a project (eg., when you quickly switch SDKs, it's written to the current workspace). Seems like this won't work with machine settings.

We'll need to decide for each setting whether it's more important that it doesn't sync across machines, or that it can be set at the folder/workspace level.

[DanTup]

I've opened microsoft/vscode#73985 to see if there's a possibility of better supporting these.

[DanTup]

Insiders now has support for machine-specific overridable settings.