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/testnvim.lua

@@ -609,12 +609,13 @@ function M._new_argv(...)
   return args, env, io_extra
 end
 
+--- Dedents string arguments and inserts the resulting text into the current buffer.
 --- @param ... string
 function M.insert(...)
   nvim_feed('i')
   for _, v in ipairs({ ... }) do
     local escaped = v:gsub('<', '<lt>')
-    M.feed(escaped)
+    M.feed(escaped) -- This also dedents :P
   end
   nvim_feed('<ESC>')
 end
@@ -812,6 +813,7 @@ function M.rmdir(path)
   end
 end
 
+--- @deprecated Use `t.pcall_err()` to check failure, or `n.command()` to check success.
 function M.exc_exec(cmd)
   M.command(([[
     try