expose event.routeId and page.routeId

[Rich-Harris]

closes #3840 (and #3907) by exposing the internal route.key as event.routeKey. This is a string based on the filename of the page/endpoint:

filename	event.routeKey
src/routes/index.svelte	""
src/routes/about.svelte	"about"
src/routes/blog/index.js	"blog"
src/routes/blog/index.svelte	"blog"
src/routes/blog/[slug].svelte	"blog/[slug]"
src/routes/blog/archive/[page=integer]/index.svelte	"blog/archive/[page=integer]"
src/routes/[...catchall]	"[...catchall]"

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[Conduitry]

Do we want to expose this in $page as well? Or are you thinking that, if someone really wanted this on the client, they could expose it in $session manually?

[Rich-Harris]

We definitely could put it in page (one of the changes in this series of PRs makes all page keys available to the client as a side effect), I was just being as conservative as possible. Do you have a use for it?

[Conduitry]

It was one of my original ideas in #269 before that turned into exposing $page.stuff. When doing navbar-type things, there are still times where routeKey would be less work than manually having to set different stuff values in each route (which is probably only made more so with shadow endpoints, if the only reason you'd need a particular load is to set stuff).

[Rich-Harris]

In that case is page.routeKey the right property, or should it just be event.key and page.key?

[Rich-Harris]

For posterity: we decided on event.routeId and page.routeId

[Rich-Harris]

merging this into #4344 to try and minimise #4354-style git fuckups