feat: add resolvePath export

[elliott-with-the-longest-name-on-github]

Closes #9934.

Adds a resolvePath export for building relative paths from route IDs and params:

resolvePath(
  `/blog/[slug]/[...somethingElse]`,
  {
    slug: 'hello-world',
    somethingElse: 'something/else'
  }
); // `/blog/hello-world/something/else`

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[dummdidumm]

Since we call it route id in other places I think it would make sense to reuse that nomenclature here

Suggested change

	export function resolvePath(id, params) {
	export function resolveRoute(id, params) {

Alternatively even resolveRouteId.

Thoughts?

[Rich-Harris]

I think resolvePath is correct — we're resolving a path from a route ID + some parameters.

That said, resolvePath sounds like path.resolve, i.e. you might expect to see resolvePath(base, relative). I don't know if there's a good name for this that doesn't sound clunky and un-SvelteKit-like.

[elliott-with-the-longest-name-on-github]

Yeah, that was my reasoning as well, Rich -- I think the current name is a good mix of what it does and brevity, and the example is pretty darn clear

[Rich-Harris]

rather than calling the function indirectly like this, we should either a) re-export it or b) put the implementation here, and use that internally instead of resolve_entry (probably better, though a more significant change)

[elliott-with-the-longest-name-on-github]

Agreed, let's put the implementation here. Makes it more clear that changing the implementation affects the public API.

[Rich-Harris]

ugh i just realised that this doesn't show up in the docs, because we also need to update ambient.d.ts

this fuels my desire to build the missing library for generating .d.ts bundles with declare module from source #9895 (comment)

[Rich-Harris]

ah i tell a lie, it's not ambient.d.ts, it's types/index.d.ts which is even worse

[Rich-Harris]

Why is the second argument optional?

[elliott-with-the-longest-name-on-github]

@Rich-Harris

The second argument isn't optional, but the values of that record are because because optional route params are optional, so if you wanted to resolvePath(id, params) with auto typed params in a route with optional params, you'd need the function to accept Record<string, string | undefined> for params.

[Rich-Harris]

ohhh i read it as Record<string, string> | undefined instead of Record<string, string | undefined>

[darrylyeo]

Woohoo! That was fast.