Add PR labeler (#2031)

## Description

It's nice to be able to filter PRs by what components they've modified.
The 5 labels that have been selected are `frontend`, `backend`,
`documentation`, `photonlib`, and `website`, since those are the primary
components in the monorepo.

## Meta

Merge checklist:
- [x] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [x] The description documents the _what_ and _why_
- [ ] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2024.3.1
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

.github/labeler.yml

@@ -0,0 +1,16 @@
+"backend":
+  - changed-files:
+      - any-glob-to-any-file: [photon-core/**, photon-server/**]
+      - all-globs-to-all-files: "!photon-server/src/main/resources/web/index.html"
+"documentation":
+  - changed-files:
+      - any-glob-to-any-file: [docs/**, photon-docs/**]
+"frontend":
+  - changed-files:
+      - any-glob-to-any-file: photon-client/**
+"photonlib":
+  - changed-files:
+      - any-glob-to-any-file: photon-lib*/**
+"website":
+  - changed-files:
+      - any-glob-to-any-file: website/**

.github/workflows/labeler.yml

@@ -0,0 +1,14 @@
+name: "Pull Request Labeler"
+on:
+  - pull_request_target
+
+jobs:
+  labeler:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@v5
+        with:
+          sync-labels: true