Add opt-in support for chimeric ProForma parsing

[josiehong]

Adds opt-in parsing support for chimeric ProForma strings using chimeric=True related to #209.

This keeps the default single-sequence behavior unchanged while allowing top-level + to separate chimeric peptidoform components when explicitly requested.

Changes

• Add chimeric option to Parser, proforma.parse, and ProForma.parse.
• Return a list of parse results / ProForma objects when chimeric=True.
• Share fixed modifications, isotope labels, and peptidoform names across chimeric components.
• Preserve the current default behavior for +, with an error message suggesting chimeric=True.
• Add tests for opt-in parsing, shared metadata, empty components, charge/adduct boundaries, and shared fixed modifications.
• Document chimeric parsing and component-wise use with existing downstream APIs.