Fix config error with missing decorations field
The latest change to window decorations (3b46859e) introduced a regression when running Alacritty without the `decorations` field specified in the configuration file. Since serde wasn't setup to fallback to the default, the complete config deserialization would fail. This resolves this issue by deserializing it to the default decorations value "Full". To make this setting a little more forgiving, this also introduces another change which ignores the case for the configuration options. So both `full` and `FuLl` are now accepted.
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