ci: configure release-please for the 2.0.0 beta line

Flagsmith/github-webhook-handler runs release-please centrally for any repo
that has release-please-config.json and .release-please-manifest.json at its
root, so no workflow is checked in here — and a .github/workflows file whose
name contains "release-please" would opt the repo back out.

`versioning: prerelease` and `prerelease: true` are both required: the
strategy strips the -beta suffix and hands back a bare major.minor.patch
unless `prerelease` is set. Together they keep the beta line on 2.0.0-beta.N
for every commit type until we flip `prerelease` off, at which point the next
release is exactly 2.0.0.

The manifest starts at the last npm release, 1.1.0, whose tag sits on the
commit below `feat!: CLI v2` — so the first release considers exactly that
commit, and Release-As pins it to a numbered beta instead of the bare
2.0.0-beta that bumpPrerelease would produce from a version with no
prerelease digits.

beep boop

Release-As: 2.0.0-beta.1

Author: khvn26

.release-please-manifest.json

@@ -0,0 +1 @@
+{".":"1.1.0"}

release-please-config.json

@@ -0,0 +1,61 @@
+{
+    "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+    "packages": {
+        ".": {
+            "release-type": "go",
+            "changelog-path": "CHANGELOG.md",
+            "versioning": "prerelease",
+            "prerelease": true,
+            "prerelease-type": "beta",
+            "draft": false,
+            "include-component-in-tag": false
+        }
+    },
+    "changelog-sections": [
+        {
+            "type": "feat",
+            "hidden": false,
+            "section": "Features"
+        },
+        {
+            "type": "fix",
+            "hidden": false,
+            "section": "Bug Fixes"
+        },
+        {
+            "type": "ci",
+            "hidden": false,
+            "section": "CI"
+        },
+        {
+            "type": "docs",
+            "hidden": false,
+            "section": "Docs"
+        },
+        {
+            "type": "deps",
+            "hidden": false,
+            "section": "Dependency Updates"
+        },
+        {
+            "type": "perf",
+            "hidden": false,
+            "section": "Performance Improvements"
+        },
+        {
+            "type": "refactor",
+            "hidden": false,
+            "section": "Refactoring"
+        },
+        {
+            "type": "test",
+            "hidden": false,
+            "section": "Tests"
+        },
+        {
+            "type": "chore",
+            "hidden": false,
+            "section": "Other"
+        }
+    ]
+}