Make LS installation silent by default

[radeksimko]

Closes #783

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Problem Statement

Whenever the user first installs the extension, or we release a new version of the language server, they're presented with a prompt such as this one:

Then after confirmation the user is further presented with a confirmation popup linking to the Changelog:

This behaviour was initially intended to make the process more transparent, based on the assumption that users want to have full control of what is being installed and when. However this seems misaligned with real VS Code user expectations and (relatedly) with how other popular VS Code extensions behave. This is also misaligned with how extensions themselves are updated in VS Code.

The default workflow involves VS Code updating extensions automatically on the background without any user interaction. In some cases (where the update requires reload) the update is staged until the user decides to click the button in Extensions pane or restarts VS Code.

Expected User Experience

Users will not receive none of the two popups by default. New versions are silently prepared for installation and installed upon next extension activation (i.e. typically with next VS Code launch).

Proposal

Any new version of the language server will be automatically (silently) installed into a staging area on the filesystem, e.g. under a separate directory ls-bin-staging within the global storage path. To avoid interrupting the user during editing the new binary will be replaced at the next extension activation, i.e. the extension will check the expected staging path upon activation.

Global VS Code settings option extensions.autoUpdate is also reflected, i.e. updating LS is entirely turned off if the user has this set to off.

(optional, depending on currently unavailable telemetry data)

Introduce a new extension config option lsChangelog: (true|false) (defaulting to false) which would cause a popup with Changelog link to be shown once a new version is installed.

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