[vscode] EnvironmentVariableMutator can now be configured

[rschnekenbu]

Vscode 1.82 has improved the support for EnvironmentVariableMutator, with options to configure when the Mutator is applied

See https://code.visualstudio.com/updates/v1_82#_configure-when-a-environmentvariablemutator-is-applied

[tsmaeder]

Since we don't have shell integration, these options don't make much sense in the Theia case. The API documentation claims that the default value for both values is false, but looking at the case, that does not seem the case: in environmentVariableCollection.ts, the value of applyAtProcessCreation seems to default to true. My proposal would be to implement applyAtProcessCreation with a default of true and to mark applyAtShellIntegration as stubbed. In this way, the current behavior is preserved and we will still have an indication that we'll have to honor the shell integration flag if we ever decide to implement that feature.