enhance: change method of getting file encoding

Currently, lualine gets the encoding using this:
[[%{strlen(&fenc)?&fenc:&enc}]] My suggestion is to change this string
to `vim.opt.fileencoding:get()` This suggestion is because I would like
to use the option "fmt = string.upper", and with the current string it
doesn't work, but using vim.opt function correctly

lua/lualine/components/encoding.lua

@@ -1,7 +1,7 @@
 -- Copyright (c) 2020-2021 hoob3rt
 -- MIT license, see LICENSE for more details.
 local function encoding()
-  return [[%{strlen(&fenc)?&fenc:&enc}]]
+  return vim.opt.fileencoding:get()
 end
 
 return encoding

lua/tests/spec/component_spec.lua

@@ -210,7 +210,13 @@ describe('Encoding component', function()
       component_separators = { left = '', right = '' },
       padding = 0,
     }
-    assert_component('encoding', opts, '%{strlen(&fenc)?&fenc:&enc}')
+    local tmp_path = 'tmp.txt'
+    local tmp_fp = io.open(tmp_path, 'w')
+    tmp_fp:write 'test file'
+    tmp_fp:close()
+    vim.cmd('e ' .. tmp_path)
+    assert_component('encoding', opts, 'utf-8')
+    os.remove(tmp_path)
   end)
 end)