This commit started out investigating a `concurrent map read write` issue, ending by replacing the map with a struct.
This is easier to reason about, and it's more effective:
```
name old time/op new time/op delta
SiteNew/Regular_Deep_content_tree-16 71.5ms ± 3% 69.4ms ± 5% ~ (p=0.200 n=4+4)
name old alloc/op new alloc/op delta
SiteNew/Regular_Deep_content_tree-16 29.7MB ± 0% 27.9MB ± 0% -5.82% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
SiteNew/Regular_Deep_content_tree-16 313k ± 0% 303k ± 0% -3.35% (p=0.029 n=4+4)
```
See #8749
Updates #7777
Added support to allow SourceMap files to be external to the build.
In addition added more information when the js compilation has an error.
Correctly append sourceMappingURL to output file.
Fix merge conflict.
Added a flag to allow turning on sourcemap in ESBuild. The current support
can only support inline or true as value for sourcemap. This is because
the way ESBuild is invoked it doesn't have a separate output path
to write the mapfile external to the asset pipeline. Add disable for "" and "0".
Add test script and make sure mage check passes.
Fixes#7607
