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/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,
         }
     }