Build extension bundle with smaller chunks for Firefox

[lukaw3d]

Related #904
Related #771

#904 replaced "background": { "scripts": with "background": { "page": so that asynchronous chunks would be loaded correctly. But it seems that our bundle is now 1 big chunk:

build/index.html                                         1.33 KB     634ms
build/favicon.1644430d.ico                               4.19 KB      59ms
build/logo192.3188d2e4.png                                1.8 KB     205ms
build/app.webmanifest                                      546 B      59ms
build/logo512.30a4930a.png                               5.36 KB     205ms
build/index.053a7c5d.js                               ⚠️  7.2 MB    21.38s
build/dump_validators.cd9b44c1.js                       67.38 KB     557ms
build/index.f7aea59f.css                                 3.15 KB     653ms
build/rubik-v20-latin-ext_latin-300.c36c8cce.woff2      20.12 KB     196ms
build/rubik-v20-latin-ext_latin-300.fa35505d.woff       26.21 KB      59ms
build/roboto-mono-v13-latin-300.7a9692de.woff2           12.4 KB     195ms
build/roboto-mono-v13-latin-300.592ef339.woff            15.2 KB      59ms

Removing .terserrc and enabling scopeHoist doesn't create more chunks or treeshake grommet-icons

By removing .terserrc beautification we could get a smaller chunk and worse stack traces

build/index.html                                          1.33 KB     641ms
build/favicon.1644430d.ico                                4.19 KB      89ms
build/logo192.3188d2e4.png                                 1.8 KB     234ms
build/app.webmanifest                                       546 B      89ms
build/logo512.30a4930a.png                                5.36 KB     235ms
build/index.2ea5eaec.js                               ⚠️  4.24 MB    21.17s
build/dump_validators.8b4f4f83.js                        66.61 KB     570ms
build/index.f7aea59f.css                                  3.15 KB     581ms
build/rubik-v20-latin-ext_latin-300.c36c8cce.woff2       20.12 KB     221ms
build/rubik-v20-latin-ext_latin-300.fa35505d.woff        26.21 KB      90ms
build/roboto-mono-v13-latin-300.7a9692de.woff2            12.4 KB     221ms
build/roboto-mono-v13-latin-300.592ef339.woff             15.2 KB      90ms

By removing .terserrc beautification and enabling scopeHoist to allow better treeshaking, we don't gain much. It creates separate slightly differently pruned entrypoints for ext and web:

build/index.html                                          1.41 KB    20.92s
build/favicon.1644430d.ico                                4.19 KB      77ms
build/logo192.3188d2e4.png                                 1.8 KB     1.55s
build/app.webmanifest                                       546 B      76ms
build/logo512.30a4930a.png                                5.36 KB     1.55s
build/index.6d97256d.js                               ⚠️  4.24 MB    20.45s
build/dump_validators.d25adec3.js                        65.67 KB    20.86s
build/index.e9b13657.css                                  3.15 KB     1.45s
build/rubik-v20-latin-ext_latin-300.c36c8cce.woff2       20.12 KB     257ms
build/rubik-v20-latin-ext_latin-300.fa35505d.woff        26.21 KB      77ms
build/roboto-mono-v13-latin-300.7a9692de.woff2            12.4 KB     257ms
build/roboto-mono-v13-latin-300.592ef339.woff             15.2 KB      77ms
build/index.0f22dd9a.js                               ⚠️  4.09 MB    20.89s
build/logo192.3188d2e4.png                                 1.8 KB     1.45s
build/dump_validators.4532afe3.js                        65.67 KB    20.86s

Add a test too