fix(tvix/eval): ' is allowed in nonfirst position in Nix identifiers

With this is_valid_nix_identifier should line up with the upstream lexer
definition:

    ID          [a-zA-Z\_][a-zA-Z0-9\_\'\-]*

While we're working on this, add a simple test checking the various
formatting rules. Interestingly, it would not be suitable as an identity
test, since you have to write

    { "assert" = null; }

in order to avoid an evaluation error, but C++ Nix is happy to print
this as

    { assert = null; }

– maybe should be considered to be a bug.

Change-Id: I0a4e1ccb5033a80f3767fb8d1c4bba08d303c5d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7744
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp

@@ -0,0 +1 @@
+{ "'quoted'" = false; "-20°" = false; "2normal" = false; "45 44 43-'3 2 1" = false; "9front" = false; Very2Normal = true; VeryNormal = true; _'12 = true; "_'12.5" = false; __internal = true; _internal = true; abort = true; assert = true; "attr.path" = false; false = true; foldl' = true; normal = true; normal2 = true; null = true; or = true; throw = true; true = true; x = true; x' = true; x'' = true; "😀" = false; }

tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.nix

@@ -0,0 +1,30 @@
+{
+  __internal = true;
+  _internal = true;
+  normal = true;
+  VeryNormal = true;
+  normal2 = true;
+  Very2Normal = true;
+  _'12 = true;
+  foldl' = true;
+  x = true;
+  x' = true;
+  x'' = true;
+
+  true = true;
+  false = true;
+  null = true;
+  or = true;
+  "assert" = true; # -ish
+  throw = true;
+  abort = true;
+
+  "9front" = false;
+  "2normal" = false;
+  "-20°" = false;
+  "45 44 43-'3 2 1" = false;
+  "attr.path" = false;
+  "'quoted'" = false;
+  "_'12.5" = false;
+  "😀" = false;
+}

tvix/eval/src/value/string.rs

@@ -168,7 +168,7 @@ fn is_valid_nix_identifier(s: &str) -> bool {
     }
     for c in chars {
         match c {
-            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' | '-' => (),
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' | '-' | '\'' => (),
             _ => return false,
         }
     }