fix: adjust to vim.validate deprecation in Nvim 0.12

[clason]

requires #3552

Closes #3499

[clason]

Note to self: dispatch outside of function definition, not inside.

[DrKJeff16]

Shouldn't the title be for Nvim 0.11? since we're doing checks against that version.

[clason]

No, because the deprecation message got added in 0.12.

[DrKJeff16]

Weird stuff, but I get it.

[clason]

nil is falsy in Lua, so we can use the same signature for both branches.

[clason]

Follow-up (not critical but forward-looking):

• replace nvim_buf_get_option() with nvim_get_option_value() (deprecated in Nvim 0.10; could use vim.bo but that is less stable API)
• replace nvim_buf_add_highlight() with vim.hl.range() or nvim_buf_set_extmark() (deprecated in Nvim 0.11)
•  replace err_writeln with utils.notify/error (deprecated in Nvim 0.11)
• pet peeve: make consistent use of vim.api or vim.lsp locals