On a large compilation unit with big templates, the total time spent in
the `PlainCharacterCursor` constructor was 470ms. This commit applies
two optimizations to reduce this time:
1. Avoid the object spread operator within the constructor, as the
generated `__assign` helper in the emitted UMD bundle (ES5) does not
optimize well compared to a hardcoded object literal. This results in a
significant performance improvement. Because of the straight-forward
object literal, the VM is now much better able to optimize the memory
allocations which makes a significant difference as the
`PlainCharacterCursor` constructor is called in tight loops.
2. Reduce the number of `CharacterCursor` clones. Although cloning
itself is now much faster because of the optimization above, several
clone operations were not necessary.
Combined, these changes reduce the total time spent in the
`PlainCharacterCursor` constructor to just 10ms.
PR Close#34332
4945274080