True two-tier syntax

The parser should fully parse expressions like:

    def expr := m`42`

And the expander should expand them into expressions like:

    def expr := astBuilder.LiteralExpr(42, _makeSourceSpan("m``", true, 1, 0, 1, 2))

Or something with similar spirit. The goal is to have AST literals be as baked into the language as ints or strings. Other quasiliterals are *not* affected by this.