gptel: Support prefixing dynamic system messages with lambda

[pabl0]

• gptel.el (gptel--parse-directive): Add optional prefix-lambda argument to prepend a lambda symbol (λ) and the first line of the documentation string, if present, to the system message.

• gptel-prefix.el (gptel--infix-add-directive): Call gptel--parse-directive' with the new prefix-lambda' arg.

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This is of course much more fun if #652 gets merged.