Rename `Lookahead.is*` function to `at*`

[ahoppen]

Similar to #1976. There are a few is* functions in Lookahead like isStartOfGetSetAccessor. For consistency these should be named at*, e.g. atStartOfGetSetAccessor.

[ahoppen]

@natikgadzhi In case you can be tempted to do this one as well.

[natikgadzhi]

Yep, I can do that today, feel free to assign it to me.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://113567634