Code Actions on save

Discussed in https://github.com/helix-editor/helix/discussions/1540

Originally posted by kevinsjoberg January 19, 2022 Helix currently supports running Code Actions manually by pressing <space>+a. Some language servers also supports running specific code actions on save, e.g., typescript-language-server and gopls.

Preferably, we extend the language server configuration to allow for specifying code actions to run on save. The actions themselves would then run asynchronously, similar to what we already do for auto formatting.