feat(cheddar): Add Sourcegraph-compatible highlighting server r/1044

Sourcegraph uses a component called syntect_server to syntax-highlight
source files.

Since we already have custom syntaxes, overrides and configuration we
might as well use them with Sourcegraph!

This implements the syntect_server "protocol" (it's just a single
endpoint) so that we can swap out the syntect_server component with
cheddar.

Note: There's a few clippy lints here, but they're being solved in a
followup commit because I wanted to take care of all of them at
once (not just the ones introduced in this change).

Change-Id: Ib518a2fa1b9fee299fe599482403599583cac479
Reviewed-on: https://cl.tvl.fyi/c/depot/+/509
Reviewed-by: lukegb <lukegb@tvl.fyi>

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