fix: `eatContextual` method and reduce keywords

[ota-meshi]

This PR fixes the eatContextual method to reduce unnecessary overriding keywords.

I noticed that the eatContextual method is not working properly. This PR fixes it.

      eatContextual(name: string) {
        if (keywordsRegExp.test(name)) {
          if (this.ts_isContextual(tokTypes[name])) {
            this.next()
            return true
          }
          return false
        } else {

-          super.eatContextual(name)
+          return super.eatContextual(name)
        }
      }

Also, by correcting the eatContextual method, I was able to reduce the overriding keywords, and in doing so I was able to remove the isContextual method from the override. This PR does that too.

[codecov-commenter]

Codecov Report

Patch coverage: 57.14% and project coverage change: -0.06 :warning:

Comparison is base (6f0f1ae) 62.56% compared to head (d280912) 62.50%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #18 +/- ## ========================================== - Coverage 62.56% 62.50% -0.06% ========================================== Files 1 1 Lines 2941 2923 -18 Branches 967 959 -8 ========================================== - Hits 1840 1827 -13 + Misses 1094 1089 -5 Partials 7 7 ``` | [Impacted Files](https://app.codecov.io/gh/TyrealHu/acorn-typescript/pull/18?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyreal+Hu) | Coverage Δ | | |---|---|---| | [src/index.ts](https://app.codecov.io/gh/TyrealHu/acorn-typescript/pull/18?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyreal+Hu#diff-c3JjL2luZGV4LnRz) | `62.50% <57.14%> (-0.06%)` | :arrow_down: |

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[ota-meshi]

I now realize that, this change fixes the bug without having to merge #17. Because async is no longer a keyword.