Add string manipulation methods to Text Value Objects

[jwillp]

This would be to make it easier to work with these types of VOs.

• startsWith
• endsWith
• contains
• replace
• matches($regex)
• length
• split
• getWords
• getChars
• toUpperCase
• toLowerCase
• concatString
• concat(OtherValueObject of the same type):
• charAt
• trim
• trimStart
• trimEnd

For all these methods it would be interesting to introduce a TextInterface, although it would not apply to chars so we might need to subdivide this concept