Configurable delay for displaying the menu

[soifou]

Feature Description

Similar to the documentation, the completion menu could benefit of an additional auto_show_delay_ms. This would be a middle ground between "always" (showing on every keystroke / min_keyword_length) and "never" (triggering manually).

From a UX perspective, this could reduces cognitive load by minimizing visual noise and display the list only during brief, intentional pauses. From a performance point of view, this could alleviate some unnecessary work.

Takes effect only if auto_show is enabled. The trigger characters should take priority over it.

Related: #446
Somewhat related: #402, #336

[hesxenon]

when completion is expensive this could also alleviate performance problems