insights: [spike] investigate methods for lines of code calculations

[leonore]

having an insight for lines of code per language across multiple repos is a common customer request.

there's ways of doing this with compute (content:output(.*\n -> $lang) count:all) and it works across small repos but there's some problems with it:

• monorepo performance is bad (doesn't work)
• performance across multiple repos is bad

this issue is about investigating whether this would be feasible to do over the filesystem (hitting gitserver) instead of search

[sourcegraph-bot-2]

Heads up @joelkw @felixfbecker @vovakulikov @unclejustin - the "team/code-insights" label was applied to this issue.

[Joelkw]

Related a bit to https://github.com/sourcegraph/sourcegraph/issues/34933