feat: add support for import assertions

[guybedford]

Adds support for parsing import assertions.

Three variables are used - "a", "as" and "ae".

For static imports these correspond to:

• "a": The start of the "assert" keyword in the static import statement
• "as": The { start of the assertion
• "ae": The } end of the assertion

For dynamic imports, "as" and "ae" are not used, instead only "a":

• "a": The index immediately after the , - so { in the dynamic import expression import('specifier', {...})
• "e": Is updated to reference the end of the specifier portion before the , in the import expression, instead of the position before the closing ). This is effectively a breaking change to the dynamic import handling.
• "ss": Remains but is now documented as the start of the dynamic import(... expression itself.
• "se": Is updated to reference the end of the import expression (position of the closing )).

Resolves #73.

[LarsDenBakker]

Thanks for this!