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/plugin/lsp/inlay_hint_spec.lua

@@ -316,13 +316,13 @@ test text
 
   local grid_without_inlay_hints = [[
   test text                                         |
-   ^                                                 |
+  ^                                                  |
                                                     |
 ]]
 
   local grid_with_inlay_hints = [[
   {1:01234}test text                                    |
-   ^                                                 |
+  ^                                                  |
                                                     |
 ]]