Pattern-matching improvements

The relational pattern allows the <, >, <=, and >= operators to appear in patterns:

string GetWeightCategory (decimal bmi) => bmi switch {
 < 18.5m => "underweight",
 < 25m => "normal",
 < 30m => "overweight",
 _ => "obese" };

With pattern combinators, you can combine patterns via three new keywords (and, or, and not):

bool IsVowel (char c) => c is 'a' or 'e' or 'i' or 'o' or 'u';

bool IsLetter (char c) => c is >= 'a' and <= 'z'
 or >= 'A' and <= 'Z';

As with the && and || operators, and has higher precedence than or. You can override this with parentheses.

The not combinator can be used with the type pattern to test whether an object is
(not) a type:

if (obj is not string) ...

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