Add environmentVariableCollection to PluginContext

[vinokurig]

Recently environmentVariableCollection was added to vscode plugin API by microsoft/vscode@af3c79e.

Feature Description:

Need to align theia's PluginContext with ExtensionContext from vscode. It is needed to use the latest vscode git extension: https://github.com/microsoft/vscode/blob/5185d7e6f8a1ca1c5e03f21d62f0a113d234a95d/extensions/git/src/terminal.ts#L20

[akosyakov]

@vinokurig Could you elaborate which extensions are using it please?

[vinokurig]

The built-in vscode Git extension uses it since 1.45.0: https://github.com/microsoft/vscode/blob/d69a79b73808559a91206d73d7717ff5f798f23c/extensions/git/src/terminal.ts#L20-L24

[amiramw]

@vinokurig are you working on this issue any time soon or we can try?

[vinokurig]

@amiramw I am working om it right now. I am going to open a PR this week

[amiramw]

@vinokurig That's great. Do you know if this the only gap in order to support builtin git?

[vinokurig]

It is already possible to use the vscode git version 1.44.2. This is needed to use later versions.