customStringToIntParser can likely be faster using a lookup table

[JensRantil]

I had a look at https://github.com/shraddhaag/1brc/blob/8513d5e70a1bfabbf46ab86a9cb6558bc9805154/main.go#L240C6-L240C29. Given that the temperatures are always within [-99.9, 99.9] and you know that it always has a tenth of accuracy, you can build up a string -> int lookup table on startup of your application. The lookup table will hold 1999 items. That way, parsing will be looking up the string in the lookup table. My gut feeling is this will be faster.

Caveat: I have not looked at the raw data file, nor have a ran the program.