Fix lexing for unterminated strings/heredocs etc.

[Earlopain]

When we hit EOF and still have lex modes left, it means some content was unterminated. Heredocs specifically have logic that needs to happen when the body finished lexing. If we don't reset the mode back to how it was before, it will not continue lexing at the correct place.

Followup to #3918. We can't call into parser_lex since it resets token locations. So I went back to goto.

Closes #3911

[Earlopain]

#{ currently gives the interpolation a statement with MissingNode. That's not correct, the missing end token leads to syntax error. It also messes up the locations when it finds the synthetic heredoc end token. I don't think there are any other tokens to consider here, should be all hopefully.

# Before
Prism.parse("\"\#{").value.statements.body[0]
=> 
@ InterpolatedStringNode (location: (1,0)-(1,3))
├── flags: newline
├── opening_loc: (1,0)-(1,1) = "\""
├── parts: (length: 1)
│   └── @ EmbeddedStatementsNode (location: (1,1)-(1,3))
│       ├── flags: ∅
│       ├── opening_loc: (1,1)-(1,3) = "\#{"
│       ├── statements:
│       │   @ StatementsNode (location: (1,1)-(1,3))
│       │   ├── flags: ∅
│       │   └── body: (length: 1)
│       │       └── @ MissingNode (location: (1,1)-(1,3))
│       │           └── flags: ∅
│       └── closing_loc: (1,3)-(1,3) = ""
└── closing_loc: ∅

# After
Prism.parse("\"\#{").value.statements.body[0]
=> 
@ InterpolatedStringNode (location: (1,0)-(1,3))
├── flags: newline
├── opening_loc: (1,0)-(1,1) = "\""
├── parts: (length: 1)
│   └── @ EmbeddedStatementsNode (location: (1,1)-(1,3))
│       ├── flags: ∅
│       ├── opening_loc: (1,1)-(1,3) = "\#{"
│       ├── statements: ∅
│       └── closing_loc: (1,3)-(1,3) = ""
└── closing_loc: ∅