upd+feat: change source file ext, add new examples

- UPD: now using `.fzs` (fromzero-source) file extension instead of
`.fzc`
- FEAT: add new examples: problematic hello world with unterminated
string and empty printline

.gitignore

@@ -1,6 +1,7 @@
 /*
 
 # Dirs
+!/docs
 !/exl
 !/include
 !/src

docs/grammar.ebnf

@@ -0,0 +1,6 @@
+program       := statement*
+statement     := expression
+expression    := call | string_literal
+call          := name "(" argument-list? ")"
+argument-list := string_literal ("," string_literal)*
+

exl/print_empty.fzs

@@ -0,0 +1 @@
+printline("")

exl/problematic_hw.fzs

@@ -0,0 +1 @@
+printline("hello)