Add string case reified types

[poteat]

Adds various case conversions.

• Add String.CamelCase to convert a string to camelCase.
• Add String.PascalCase to convert a string to PascalCase.
• Add String.SnakeCase to convert a string to snake_case.
• Add String.KebabCase to convert a string to kebab-case.
• Add String.ConstantCase to convert a string to CONSTANT_CASE.
• Reify String.Capitalize to a value-level function.
• Add String.Words to split a string into words.
• Add String.IsUppercaseLetter to check if a string is uppercase.
• Add String.IsLowercaseLetter to check if a string is lowercase.
• Add String.IsDigit to check if a string is a digit.