[Feature] String Parsing

[gluax]

🚀 Feature

The goal is to add string as a Token and for it to be parsable to AST nodes.
Depends on #943.

Motivation

This helps the implementation goal of #929.

Implementation

Tokenizer Side

• A new String Token Enum should be created leo/parser/src/tokenizer/token.rs. This should go under the commented Types section.
• A new StringLit Token Enum should be created leo/parser/src/tokenizer/token.rs. This should go under the commented Types section.

Parser Side

• In parse_primary_expression should match on Token::StringLit and then parse the string to a AST Node ValueExpression leo/parser/src/parser/expression.rs.

Tests

• Parser Tests:
  • Escaped Characters inside the String.
  • Hex Characters(although I suppose these could be grouped with the prior) inside the String.
  • Valid String Syntax.
  • Invalid String Syntax.

[damirka]

Closed by #957.