Add gptel--get-prompt-from-region function

[eval-exec]

When using a region as a prompt, I suggest extract it to independent function:gptel--get-prompt-from-region.

Then user will be convinient to hack the function.
For example, I can add markdown code fence symbol around prompt by:

  (defun exec/major-mod-to-markdown-lang()
	(let* ((major
			(car (split-string (symbol-name major-mode) "-"))))
	  (cond ((string= major "emacs")
			 "elisp")
			(t major))))

  (require 'gptel-transient)

  (defun gptel--get-prompt-from-region()
	"Get the prompt from the region."
	(format "\n```%s\n%s\n```\n"
			;; major mode symbol, if rust-ts-mode, then rust
			(exec/major-mod-to-markdown-lang)
			(buffer-substring-no-properties
			 (region-beginning) (region-end))))