Improve parse-error messages

[mhasel]

Currently our parser errors could do with some improvement. Take the follwing example, where I am trying to declare a variable with a reserved keyword as name:

FUNCTION main : DINT
VAR
    by  : BYTE;
    to : DINT;
END_VAR
    // do stuff
END_FUNCTION

Upon compilation, we print the following error:

error: Unexpected token: expected KeywordEndVar but found 'by  : BYTE;
    to : DINT;'
  ┌─ target/demo.st:3:5
  │  
3 │ ╭     by  : BYTE;
4 │ │     to : DINT;
  │ ╰──────────────^ Unexpected token: expected KeywordEndVar but found 'by  : BYTE;
    to : DINT;'

This message isn't really helpful for the end-user, since it in no way points them to the variable name being the issue. We should try to make our parser errors more user-friendly.