split per-type registration points into per-type files

- move README per-type sections into docs/<type>/{remote,local,virtual}.md
- split provider.go capability and metadata types into cap_*.go and metadata_*.go
- register providers via internal/provider/all, one file per type
- declare each package type in its own file, registering via init()
- split the e2e caching table and UI type colours/snippets per type

docs/helm/remote.md

@@ -0,0 +1,17 @@
+# `helm` remote
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `helm` | `index.yaml` | `.tgz` charts |
+
+## Terraform
+
+```hcl
+resource "artifactapi_remote_helm" "jetstack" {
+  name     = "jetstack"
+  base_url = "https://charts.jetstack.io"
+
+  immutable_ttl = 0
+  mutable_ttl   = 3600
+}
+```