(fs) Add methods to Files for reading/writing a string from/to a file

Reading the content of a file into a string and writing a string text to a file are common operations. This enhancement suggests adding the following methods:

String Files.readString(Path path)
String Files.readString(Path, Charset)

Path Files.write(Path, CharSequence, OpenOption...)
Path Files.write(Path, CharSequence, Charset, OpenOption...)

In addition, a utility method that compares two files:
Files.isSameContent(path1, path2)

There will be a further research on String operations as relevant to the File operations to determine whether there are legitimate alternative approaches to the above. For example, specifying start/end index to limit the range to be read, which allows applications to control the size of the returning string. Or, treating the content of a file as having sections, and therefore return a list of strings instead.