fix: don't use info nodes before cursor for completion

[mhuisi]

This fixes an issue where the completion would use info nodes before the cursor for computing completions.

Fixes #3462.

ToDo:

• Fix test failures for completions that previously worked by accident (cc: @Kha)
• stage0 update

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase e8a2786d6d4defa29f8991eb03e1dd975311e5ae --onto 4c0106d757d4d6d3b7f3903611ce22100d539d2a. (2024-03-27 15:45:03)
• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 70924be89c3aa7eee7dd0e115bf3b9df2d96eb78 --onto b4caee80a3dfc5c9619d88b16c40cc3db90da4e2. (2024-03-27 17:40:14)