Improve error message for identifiers with leading underscore

[mohammadfawaz]

Original PR: #28897

When users write identifiers starting with underscore, (e.g.: let _c: u32), the parser produces an uninformative error.

Root cause of that is that lexer splits _c into two separate tokens, and the parser had no grammar rule matching the pattern, so LARLPOP generates a generir error listing what tokens it expected at that position (which happened to be '(' for tuple destructuring).

So, this adds new lexer token - InvalidLeadingUnderscoreIdent that catches any leading underscore next to identifiers.

Before:

Error [EPAR0370005]: expected an identifier, '(' -- found '_'
    --> \\?\C:\Users\tetektoza\source\repos\leo\test_underscore_project\test_underscore\src\main.leo:3:13
     |
   3 |         let _c: u32 = a + b;
     |             ^

After:

Error [EPAR0370047]: Identifier cannot start with an underscore
    --> \\?\C:\Users\tetektoza\source\repos\leo\test_underscore_project\test_underscore\src\main.leo:3:13
     |
   3 |         let _c: u32 = a + b;
     |             ^^
     |
     = Identifiers must start with a letter.

Resolves: #28755