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/ui/screen.lua

@@ -494,9 +494,8 @@ function Screen:expect(expected, attr_ids, ...)
 
   local expected_rows = {} --- @type string[]
   if grid then
-    -- Remove the last line and dedent. Note that gsub returns more then one
-    -- value.
-    grid = dedent(grid:gsub('\n[ ]+$', ''), 0)
+    -- Dedent (ignores last line if it is blank).
+    grid = dedent(grid, 0)
     for row in grid:gmatch('[^\n]+') do
       table.insert(expected_rows, row)
     end