Invalid event severity detection for weatheralerts

[MrBartusek]

The current implementation of weatheralerts integration relies on severity attribute rather than detecting if the alert is a warning, advisory, watch or statement this can produce invalid alert severity (#144 - severity: Extreme, Tornado Watch). This integration should rely on detecting last word of alert like env_canada:

https://github.com/MrBartusek/MeteoalarmCard/blob/master/src/integrations/env_canada.ts#L244-L255