Fix gcc warning -Wmissing-field-initializers

The approach that was taken here was to add default values to the type
definitions rather than specify them whenever they are missing.

Now the only remaining warning is '-Wunused-parameter' which @jade said
is usually counterproductive and that we can just disable it:
https://git.lix.systems/lix-project/lix/issues/456#issuecomment-6617

So this change adds the flags '-Wall', '-Wextra' and
'-Wno-unused-parameter', so that all warnings are enabled except for
'-Wunused-parameter'.

Change-Id: Ic223a964d67ab429e8da804c0721ba5e25d53012

src/libexpr/parser/state.hh

@@ -9,7 +9,7 @@ namespace nix::parser {
 struct StringToken
 {
     std::string_view s;
-    bool hasIndentation;
+    bool hasIndentation = false;
     operator std::string_view() const { return s; }
 };