[SciPy]: Tokenize `ASSIGN TO`

[Pranavchiku]

on executing lfortran --show-asr --implicit-typing ../scipy/scipy/special/cdflib/dinvr.f results in

tokenizer error: Expecting terminating symbol for subroutine
   --> /home/pranavchiku/scipy/scipy/special/cdflib/dinvr.f:100:7
    |
100 |       ASSIGN 10 TO i99999
    |       ^ 


Note: if any of the above error or warning messages are not clear or are lacking
context please report it to us (we consider that a bug that needs to be fixed).

[Smit-create]

Similar to: #672

[konradha]

@Smit-create Well, these issues aren't really alike. Enabling CONTINUE is catching the edge cases and then we're good; enabling ASSIGN TO turns out to be enabling another kind of GOTO while also implementing it in ASR :)

[konradha]

@Smit-create FYI I enabled CONTINUE within a procedure's body; you might want to have a look at the PR. If there are more cases with CONTINUE please ping me :)

[konradha]

#848