feat: add check keyword and new assignment syntax

crates/lang/src/parser/lexer.rs

@@ -63,6 +63,7 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
     let keyword = text::ident().map(|s: String| match s.as_str() {
         "as" => Token::As,
         "assert" => Token::Assert,
+        "check" => Token::Assert,
         "const" => Token::Const,
         "fn" => Token::Fn,
         "if" => Token::If,

crates/lang/src/parser/token.rs

@@ -58,6 +58,7 @@ pub enum Token {
     // Keywords (alphabetically):
     As,
     Assert,
+    Check,
     Const,
     Fn,
     If,
@@ -130,6 +131,7 @@ impl fmt::Display for Token {
             Token::EmptyLine => "EMPTYLINE",
             Token::As => "as",
             Token::Assert => "assert",
+            Token::Check => "check",
             Token::When => "when",
             Token::Is => "is",
             Token::Const => "const",