fix(ssr): stream mode throw error in exportStatic

[ycjcl868]

• fix error when ssr.mode = 'stream' and exportStatic
• replace fs sync methods into asynchronous in ssr. refs: fs.promises.readFile is 40% slower than fs.readFile nodejs/node#37583

Close https:/lifegit/myapp2/issues/1

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https:/umijs/umi/ISSUE_URL

[codecov]

Codecov Report

Merging #6510 (3a47ae1) into master (31f2372) will increase coverage by 0.02%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #6510      +/-   ##
==========================================
+ Coverage   84.87%   84.89%   +0.02%     
==========================================
  Files         157      157              
  Lines        3590     3596       +6     
  Branches      988      989       +1     
==========================================
+ Hits         3047     3053       +6     
  Misses        533      533              
  Partials       10       10              

Impacted Files	Coverage Δ
...eset-built-in/src/plugins/features/exportStatic.ts	28.57% <0.00%> (ø)
...es/preset-built-in/src/plugins/features/ssr/ssr.ts	44.06% <66.66%> (ø)
...ilt-in/src/plugins/features/ssr/templates/utils.ts	100.00% <100.00%> (ø)
packages/preset-built-in/src/plugins/utils.ts	100.00% <100.00%> (ø)

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