fix(core): allow lowercase locale prefixes

[dphang]

Related: #1478

[slsnextbot]

Handler Size Report

There are changes to handler sizes. Please review.

Base Handler Sizes (kB) (commit 9a07c65)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1227,
            "Minified": 433
        },
        "API Lambda": {
            "Standard": 86,
            "Minified": 34
        },
        "Image Lambda": {
            "Standard": 894,
            "Minified": 354
        },
        "Regeneration Lambda": {
            "Standard": 620,
            "Minified": 220
        }
    }
}

New Handler Sizes (kB) (commit a5b5898)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1227,
            "Minified": 433
        },
        "API Lambda": {
            "Standard": 86,
            "Minified": 34
        },
        "Image Lambda": {
            "Standard": 895,
            "Minified": 354
        },
        "Regeneration Lambda": {
            "Standard": 620,
            "Minified": 220
        }
    }
}

[codecov]

Codecov Report

Merging #1496 (8782be1) into master (9a07c65) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 8782be1 differs from pull request most recent head a5b5898. Consider uploading reports for the commit a5b5898 to get more accurate results

@@            Coverage Diff             @@
##           master    #1496      +/-   ##
==========================================
+ Coverage   84.14%   84.17%   +0.02%     
==========================================
  Files          97       97              
  Lines        3381     3387       +6     
  Branches     1009     1009              
==========================================
+ Hits         2845     2851       +6     
  Misses        476      476              
  Partials       60       60              

Impacted Files	Coverage Δ
packages/libs/core/src/route/locale.ts	99.06% <100.00%> (+0.05%)	⬆️

---

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 9a07c65...a5b5898. Read the comment docs.