lua-mode: Update to 20210802

Changelog from https://github.com/immerrr/lua-mode/tags

v20210802
* performance optimizations for automatic indentation
  * lua-skip-ws-and-comments-forward, lua-skip-ws-and-comments-backward: add new
    functions
  * lua-comment-start-pos: make sure point inside comment starter "--" is
    considered inside comment
  * lua-funcname-at-point: refactor to avoid copying syntax table to improve
    performance
  * lua--signum: add backward compat helper
  * code cleanups & minor refactors

(nikita)