First query in a process takes 9 ms to compile, half of it building ANTLR character sets #1301

This work was produced with the help of language models.

The first query in a process spends about half of its compile time deserializing the lexer's ATN: RETURN 1 compiles in 9.4 ms the first time and in under 0.1 ms afterwards (the 0.18 ms in #1290 is a later query). The vendored ANTLR runtime's IntervalSet::add walks the whole interval list for every interval it adds, and the lexer's Unicode character sets are built one sorted interval at a time, so building a large set is quadratic in its number of intervals.

Expected: building a set from sorted, non-touching intervals is linear in its size. Actual: each added interval scans every interval already in the set.

Part of #1290. Fix in #1302.