Weird successfull parsing of invalid path literals

[sternenseemann]

Describe the bug

Was messing around with invalid tilde starting path literals:

• ~foo is parsed to Ident(Ident(NODE_IDENT@2..4)), the ~ seemingly disappears into nowhere. The ident is retained as oo, i.e. in tvix you can evaluate let oo = 42; in ~foo. This literal should be a parse failure.
• ~foo/bar is parsed to Path(Path(NODE_PATH@2..8)), yielding the path oo/bar. This should also be a parse failure.

Nix forbids home path literals that don't start with ~/, so both expressions are invalid Nix. Probably significant out two chars are dropped from the literals and ~/ is also two chars long.

[tazjin]

I'm guessing this is a duplicate of #146. It should be possible to have rnix report these as errors