submodule: convert versions/26.00 from committed vendor to git submodule

Remove the ~1315 vendor source files committed under versions/26.00/ and
replace them with a git submodule pointing to ip7z/7zip tag 26.00
(SHA 839151e).  The upstream 26.00 tag does not ship a Nsis7z bundle, so
all project-owned build files live in the separate versions/26.00-bundle/
directory (added in the next commit).

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "versions/26.01"]
 	path = versions/26.01
 	url = https://github.com/ip7z/7zip.git
+[submodule "versions/26.00"]
+	path = versions/26.00
+	url = https://github.com/ip7z/7zip.git