Fix parser arrow functions

[yyc]

Realized I was testing the wrong thing in #107.
Fixed the errors and wrote correct tests this time (see mci-parser.ts)

@martin-henz please check whether the output in https://github.com/source-academy/js-slang/blob/c58006ca36867e79f1819d2c99d953bf1af7ddab/src/__tests__/__snapshots__/mci-parser.ts.snap is what's expected.

Should work with docker after merging.

[coveralls]

Pull Request Test Coverage Report for Build 253

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+9.6%) to 52.423%

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Totals
Change from base Build 244:	9.6%
Covered Lines:	988
Relevant Lines:	1760

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💛 - Coveralls

[remo5000]

LGTM. Works on docker