Support for Multiline Placeholders

[caring-coder]

The office-stamper library does not support multiline placeholders. This makes it difficult to pass multiline string variables to custom functions.

Add support for multiline placeholders, allowing multiline string variables to be used within custom functions.

I've considered flattening multiline strings with delimiters and splitting them in custom functions, but this approach is cumbersome and error-prone.

Supporting multiline placeholders would enhance the library's usability. For example:

Hello,

${CustomFunction("This is an example
of a multiline
string.")}

Regards,

[caring-coder]

Support for line breaks for now in 2.4