Adds an `index` command as suggested by #7186.
> Future enhancement suggestion: include ... index() for block indices
> within these repeated blocks. Would be very useful for string tables
> in the example ref.
This makes it easier to write a binary file which contains offsets and
size values. Using this for unit tests for the LDML keyboard compiler.
Example kmldmlc basic.txt is updated and included.
Future enhancement suggestion: include count() for repeated blocks and
index() for block indices within these repeated blocks. Would be very
useful for string tables in the example ref.
