[Backport release-0.9] fix(fs): make normalize() work with '/' path (#24060)

fix(fs): make `normalize()` work with '/' path

Problem: Current implementation of "remove trailing /" doesn't
account for the case of literal '/' as path.
Solution: Remove trailing / only if it preceded by something else.

(cherry picked from commit 80ff66118a)

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>

test/functional/lua/fs_spec.lua

@@ -270,6 +270,12 @@ describe('vim.fs', function()
     it('works with backward slashes', function()
       eq('C:/Users/jdoe', exec_lua [[ return vim.fs.normalize('C:\\Users\\jdoe') ]])
     end)
+    it('removes trailing /', function()
+      eq('/home/user', exec_lua [[ return vim.fs.normalize('/home/user/') ]])
+    end)
+    it('works with /', function()
+      eq('/', exec_lua [[ return vim.fs.normalize('/') ]])
+    end)
     it('works with ~', function()
       eq( exec_lua([[
       local home = ...