feat!: turn on experimental selector inference by default

[idoros]

This PR flips the default, so that the experimentalSelectorInference flag is on unless explicitly set to false - the false behavior is kept for now, but will be removed in the future within version 6.

• set default to true
• report false as unsupported mode that will be removed in a future version
• change test expectations that relay on the old behavior
• refactored tests to match new default (kept some experimentalSelectorInference=false to test previous behavior)

As part of the upgrade in the vscode extension, a task must be undertaken to verify the installed @stylable/core version and, if not configured by the user, override the default experimentalSelectorInference setting according to the detected version (false for <=5, true for >=6).