Fix parsing of import statement

[mattjohnsonpint]

Parsing a top-level import statement should work, but fails with the following:

AssertionError: assertion failed
      at Y.assert (file:///Users/matt/Code/visitor-as/node_modules/assemblyscript/dist/assemblyscript.js:9:3029)
      at Kn.parseImport (file:///Users/matt/Code/visitor-as/node_modules/assemblyscript/dist/assemblyscript.js:17:124365)
      at Kn.parseTopLevelStatement (file:///Users/matt/Code/visitor-as/node_modules/assemblyscript/dist/assemblyscript.js:17:102113)
      at SimpleParser.parseTopLevelStatement (file:///Users/matt/Code/visitor-as/dist/simpleParser.js:24:33)
      at Context.<anonymous> (file:///Users/matt/Code/visitor-as/__tests__/tests.spec.js:166:20)
      at process.processImmediate (node:internal/timers:478:21)

This PR adds a test for that, and fixes it by setting the parser's currentSource property.

I'm not certain if the same fix is required for the other parsing methods or not. I only found the problem while trying to parse an import statement. LMK if you want me to make the same fix to the other parsing methods. Thanks.

[willemneal]

Makes sense to me! Thanks! @jtenner Are you still maintaining this?

[mattjohnsonpint]

Looks like @jtenner has been away from GitHub for some time now. No worries. I have a workaround. I just didn't want to this to get lost, for whenever he returns.