in a plain text file, cursor before a closing bracket, type an opening
one and nothing autocloses. `anything|)` + `[` gives `anything[|)` not
`anything[]|)`. fine in js/css/py.

autoclose checks the language's `autoclose_before` set. every language
sets one, plain text didn't, so a closing bracket ahead of the cursor
killed it. gave it `)]}`.

went `)]}` not the usual `;:.,=}])>` since plain text only has brackets.
`.typ` (Typst) is a separate extension, needs the same fix there.

Fixes #60560.

Release Notes:

- Fixed nested brackets not auto-closing in plain text files