Add string assertion "MustBeTrimmed"

[feO2x]

We should add the following assertions for string, ReadOnlySpan<char> and Span<char>:

• IsTrimmed: checks if a character sequence starts and ends with a non-whitespace character, returns true or false.
• MustBeTrimmed: same as IsTrimmed, but throws a StringException.
• IsTrimmedAtStart: checks if a character sequence starts with a non-whitespace character, returns true or false.
• MustBeTrimmedAtStart: same as IsTrimmedAtStart, but throws a StringException.
• IsTrimmedAtEnd: checks if a character sequence ends with a non-whitespace character, returns true or false.
• MustBeTrimmedAtEnd: same as IsTrimmedAtEnd, but throws a StringException.

[feO2x]

We can leave out the Span<T> overloads because there is an implicit conversion from Span<T> to ReadOnlySpan<T>.