Hide files under `ephemeral` from text search

[helin24]

Context from original request to VS Code extension: Dart-Code/Dart-Code#5776

[FMorschel]

Just to add this as context here too, the changes made on Dart Code were:

"files.exclude": {
	"**/flutter/ephemeral": true,
	"**/.symlinks": true,
	"**/.plugin_symlinks": true
},
"search.exclude": {
	"**/.dart_tool": true
},

For more context, see the comment at Dart-Code/Dart-Code#5776 (comment).

[DanTup]

Some additional info that might not be obvious:

• These were contributed settings in Dart-Code. These settings get merged with the base VS Code defaults, but a user can override them (by adding them to their settings with a value false). We're hiding these things by default, but the user can still restore any of them.
• files.exclude excludes from both explorer and search results
• search.exclude excludes only from search results

The goal here was to avoid the generated files in those locations from showing up in places like search, but still allow the user to revert to seeing them if they wanted. I personally have .dart_tool in my own files.exclude too (since I don't see value in it cluttering my explorer), but it's less clear to me that everyone would want that so I left it in explorer for now (I'd happily revisit that based on feedback though).