feat: add eofNewLine config

[dsherret]

Closes #22

Adds an eofNewLine option to control the newline at the end of the file:

• "always" (default) - always end the file with a newline. This is the existing behaviour.
• "maintain" - end the file with a newline only when it already ends with one.
• "never" - never end the file with a newline.

This is useful for files like package.json, where package managers often remove the trailing newline when adding or removing packages.

{
  "json": {
    "eofNewLine": "maintain"
  }
}

• maintain checks the text after the BOM is stripped and the package.json conventions are applied (the conventions keep what trails the root value). It counts a \n or a lone \r anywhere in the trailing whitespace.
• The newline that's written still follows newLineKind.