Declare character code constants per file

[adamwathan]

This PR effectively reverts #13497 but preserves the naming improvements that makes the name of each constant consistent between files.

I noticed when benchmarking segment that objects access like Token.OPEN_CURLY made the function 25x slower (!) so we should at a minimum import all of the character codes directly instead of using import * as Token from ....

I feel like since these aren't application-specific constants there's no harm in just redeclaring them in each file anyways — BACKSLASH is going to be 0x5c until the end of time, there's no risk that we have to update that value and now have to update it in more than one place. This also reduces the number of dependencies between files which is nice if we ever wanted to publish anything here as a separate package for any reason.

[RobinMalfait]

A note for future reference: this 25x slowdown only happens when running benchmarks and tests on the raw source code. The final dist files don't have this issue because the constants are inlined.