feature: add impl-source-read-only editor-variable

[sakurawald]

To close issue: #1615

This option is enabled by default, to disable it, eval the form in init.lisp

(setf (lem:variable-value 'lem-core::impl-source-read-only :global) nil)

The typical value of source dir for sbcl implementation are:

• /usr/share/sbcl-source/src/code/ (installed via yay -S sbcl)
• ~/.roswell/src/sbcl-2.4.10/src/code/ (installed via roswell)

There may be a better function to decide if (current-directory) is part of implementation source directory.

[sakurawald]

Another way to get the source directory at runtime using the running image is to use the SYS:SRC logic path.

The example function:

CL-USER> (pathname-match-p "/usr/share/sbcl-source/src/" (second (assoc "SYS:SRC;**;*.*.*" (logical-pathname-translations "SYS") :test 'equal)))

T