feat(lua): vim.text.indent()

Problem:
Indenting text is a common task in plugins/scripts for
presentation/formatting, yet vim has no way of doing it (especially
"dedent", and especially non-buffer text).

Solution:
Introduce `vim.text.indent()`. It sets the *exact* indentation because
that's a more difficult (and thus more useful) task than merely
"increasing the current indent" (which is somewhat easy with a `gsub()`
one-liner).

test/functional/lua/secure_spec.lua

@@ -153,16 +153,16 @@ describe('vim.secure', function()
       feed('v')
       screen:expect {
         grid = [[
-          ^let g:foobar = 42                                                             |
-        {1:~                                                                               }|*2
-        {2:]]
+          ^let g:foobar = 42                                                               |
+          {1:~                                                                               }|*2
+          {2:]]
           .. fn.fnamemodify(cwd, ':~')
           .. pathsep
           .. [[Xfile [RO]{MATCH:%s+}}|
-                                                                                        |
-        {1:~                                                                               }|
-        {4:[No Name]                                                                       }|
-                                                                                        |
+                                                                                          |
+          {1:~                                                                               }|
+          {4:[No Name]                                                                       }|
+                                                                                          |
       ]],
       }