parser: flag error for ambiguous use of semicolons

[simonvonhackewitz]

fix #2702

Examples for ambiguous use of semicolon

my_feat => say "Hi"; say "ambiguous"

if true if true then say "OK"; say "ambiguous"

Whats still OK

a := 1; say "declared a"

if true then yak "this"; say " is fine";

[simonvonhackewitz]

NYI: code for the suggested corrections in the error message is still missing