Update prefixed strings

[savq]

• Make prefixed_string_literal and prefixed_command_literal primary expressions instead of literals.
• Allow quotables and prefixed strings in coefficient expressions.
• Allow _primary_expression in any type position (type arguments, RHS of typed expressions, etc).
• Fix character_literal allowing trailing whitespace.
• Fix string_interpolation allowing whitespace after $.
• Remove 5 files from known-failures.

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This could've gone in #72 but whatever.

closes #39 and closes #55 (duplicate)