feat(lint): add stderr and ignore_exit_code options

Some linters (e.g., cpplint) output to stderr and exit non-zero when
lint issues are found, which guard treats as a failure. This adds two
options to handle such tools:

- `stderr`: capture stderr instead of stdout
- `ignore_exit_code`: don't treat non-zero exit as error

Example usage:
```lua
ft('cpp'):lint({
    cmd = 'cpplint',
    fname = true,
    stderr = true,
    ignore_exit_code = true,
    parse = ...,
})
```

Author: barrettruth

lua/guard/_meta.lua

@@ -28,6 +28,8 @@
 ---@field env table<string, string>?
 ---@field timeout integer?
 ---@field health function?
+---@field stderr boolean?
+---@field ignore_exit_code boolean?
 
 ---@alias LintConfig LintConfigTable|fun(): LintConfigTable
 

lua/guard/lint.lua

@@ -12,7 +12,7 @@ local custom_ns = {}
 ---@async
 ---@param cmd string[]
 ---@param cwd string
----@param config {env: table?, timeout: integer?}
+---@param config {env: table?, timeout: integer?, stderr: boolean?, ignore_exit_code: boolean?}
 ---@param input string|string[]
 ---@return string output
 ---@return {code: integer, stderr: string, cmd: string}? error
@@ -31,15 +31,15 @@ local function exec_linter(cmd, cwd, config, input)
     handle:write(nil)
   end)
 
-  if result.code ~= 0 and #result.stderr > 0 then
+  if not config.ignore_exit_code and result.code ~= 0 and #result.stderr > 0 then
     return '', {
       code = result.code,
       stderr = result.stderr,
       cmd = cmd[1],
     }
   end
 
-  return result.stdout, nil
+  return config.stderr and result.stderr or result.stdout, nil
 end
 
 ---@param buf number?

lua/guard/util.lua

@@ -176,6 +176,8 @@ function M.toolcopy(c)
     timeout = c.timeout,
     parse = c.parse,
     health = c.health,
+    stderr = c.stderr,
+    ignore_exit_code = c.ignore_exit_code,
   }
 end