fix(abbr): don't stop abbr on `{}()`

[MithicSpirit]

Since open braces/parentheses are almost always preceded by spaces, and close braces/parentheses are almost always succeeded by spaces, there's no real advantage to immediately converting an abbreviation when these are seen. Moreover, this meant that the abbreviations \{{ and \}} were impossible to use (and we had to resort to \c[ and \c] instead). Overall, this change feels like a net positive (and users can still override by just pressing <Tab> anyways.

This also makes a couple of minor changes with no noticeable effect

• Passing in {syntax} to vim.lsp.util.open_floating_preview(), which is documented as a mandatory parameter.
• Not returning vim.schedule() calls in insert_char_pre(), since returning truthy values from an autocmd callback causes the autocmd to be deleted. vim.schedule() is documented as always returning nil, so this doesn't really matter, but this is clearer.

[Julian]

Thanks. I honestly don't recall why we special cased these, but this seems OK to me (and the tests pass after a rebase). Merging, appreciated.