Don't emit await keyword with ES transforms disabled

[alangpierce]

Follow-up to #623. With transforms disabled, we should just early return even in
the async case.

[codecov]

Codecov Report

Merging #624 (c643b00) into main (edc56e0) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
+ Coverage   82.18%   82.20%   +0.02%     
==========================================
  Files          56       56              
  Lines        5876     5874       -2     
  Branches     1327     1326       -1     
==========================================
  Hits         4829     4829              
+ Misses        763      762       -1     
+ Partials      284      283       -1     

Impacted Files	Coverage Δ
src/TokenProcessor.ts	93.75% <ø> (+1.64%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update edc56e0...c643b00. Read the comment docs.

[github-actions]

Benchmark results

Before this PR: 340.8 thousand lines per second
After this PR: 346 thousand lines per second

Measured change: 1.53% faster (3.64% slower to 6.89% faster)
Summary: Likely no significant difference