Zephyr timing implementation refactored into two separate files

[erlingrj]

Clean up Zephyr platform implementation:

• Remove the use of preprocessor to conditionally compile the two different timing APIs
• Implement the kernel timing API and counter timing API in two separate files
• remove the _lf_ prepending of all the static local variables. They are static and local to that file, so no need to add long complicated variable names that are hard to parse
• Allow the user to pick which Timing API to use by setting the flag LF_ZEPHYR_CLOCK_COUNTER or LF_ZEPHYR_CLOCK_KERNEL. If no such flag is defined then we pick what is available but we prefer the counter version
• Let the user set LF_ZEPHYR_STACK_SIZE and LF_ZEPHYR_THREAD_PRIORITY to configure stack sizes and priorities of the threads created. Defaults to 1024 and 5 if nothing is chosen.

Closes lf-lang/lingua-franca#2063

[lhstrh]

Nice! Looks great to me. I guess this doesn't require any changes in the code generator?

[erlingrj]

Correct, no changes required in lingua franca