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).

src/gen/gen_eval_files.lua

@@ -766,18 +766,8 @@ local function scope_more_doc(o)
 end
 
 --- @param x string
---- @return string
 local function dedent(x)
-  local xs = split(x)
-  local leading_ws = xs[1]:match('^%s*') --[[@as string]]
-  local leading_ws_pat = '^' .. leading_ws
-
-  for i in ipairs(xs) do
-    local strip_pat = xs[i]:match(leading_ws_pat) and leading_ws_pat or '^%s*'
-    xs[i] = xs[i]:gsub(strip_pat, '')
-  end
-
-  return table.concat(xs, '\n')
+  return (vim.text.indent(0, (x:gsub('\n%s-([\n]?)$', '\n%1'))))
 end
 
 --- @return table<string,vim.option_meta>

src/gen/gen_help_html.lua

@@ -148,10 +148,6 @@ local function url_encode(s)
   )
 end
 
-local function expandtabs(s)
-  return s:gsub('\t', (' '):rep(8)) --[[ @as string ]]
-end
-
 local function to_titlecase(s)
   local text = ''
   for w in vim.gsplit(s, '[ \t]+') do
@@ -275,25 +271,13 @@ end
 ---
 --- Blank lines (empty or whitespace-only) are ignored.
 local function get_indent(s)
-  local min_indent = nil
-  for line in vim.gsplit(s, '\n') do
-    if line and not is_blank(line) then
-      local ws = expandtabs(line:match('^%s+') or '')
-      min_indent = (not min_indent or ws:len() < min_indent) and ws:len() or min_indent
-    end
-  end
-  return min_indent or 0
+  local _, indent = vim.text.indent(0, s, { expandtab = 8 })
+  return indent
 end
 
 --- Removes the common indent level, after expanding tabs to 8 spaces.
 local function trim_indent(s)
-  local indent_size = get_indent(s)
-  local trimmed = ''
-  for line in vim.gsplit(s, '\n') do
-    line = expandtabs(line)
-    trimmed = ('%s%s\n'):format(trimmed, line:sub(indent_size + 1))
-  end
-  return trimmed:sub(1, -2)
+  return vim.text.indent(0, s, { expandtab = 8 })
 end
 
 --- Gets raw buffer text in the node's range (+/- an offset), as a newline-delimited string.