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).

runtime/lua/vim/health.lua

@@ -186,18 +186,13 @@ local function get_healthcheck(plugin_names)
   return healthchecks
 end
 
---- Indents lines *except* line 1 of a string if it contains newlines.
+--- Indents lines *except* line 1 of a multiline string.
 ---
 --- @param s string
 --- @param columns integer
 --- @return string
 local function indent_after_line1(s, columns)
-  local lines = vim.split(s, '\n')
-  local indent = string.rep(' ', columns)
-  for i = 2, #lines do
-    lines[i] = indent .. lines[i]
-  end
-  return table.concat(lines, '\n')
+  return (vim.text.indent(columns, s):gsub('^%s+', ''))
 end
 
 --- Changes ':h clipboard' to ':help |clipboard|'.