fix(lambda-at-edge): locale subpath bug fixes

[dphang]

• SSG index page routing
• static pages / json files only in locale directories
• separate unit tests for locale testing with/without basepath

Related: #721

[codecov]

Codecov Report

Merging #888 (95e000f) into master (8e39294) will increase coverage by 0.35%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
+ Coverage   80.36%   80.72%   +0.35%     
==========================================
  Files          64       64              
  Lines        2394     2402       +8     
  Branches      550      560      +10     
==========================================
+ Hits         1924     1939      +15     
+ Misses        407      399       -8     
- Partials       63       64       +1     

Impacted Files	Coverage Δ
...es/libs/lambda-at-edge/src/routing/locale-utils.ts	95.83% <87.50%> (-4.17%)	⬇️
packages/libs/lambda-at-edge/src/build.ts	90.18% <92.30%> (+0.99%)	⬆️
...ackages/libs/lambda-at-edge/src/default-handler.ts	93.37% <100.00%> (+1.62%)	⬆️

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