Design a programming language-agnostic text processor that can handle: 1) Nested string delimiters (e.g., "quotes 'inside' quotes") 2) Escaped delimiters (e.g., backslash escaping) 3) Multiline string literals 4) Interpolation within delimiters 5) Custom delimiter definitions 6) Mixed delimiter types in same document. Your solution should include: state machine design, formal grammar, implementation pseudocode, and handling of edge cases. Test with sample inputs including nested delimiters at least 3 levels deep.