chore(format): adds prettier and commands to run it

... including ci/cd check-only commands.

The code is quite complex and missing a lot of dev-ops types checks and
standards. One of the key problems with codebases like this is
on-boarding new developers to the codebase (like myself). When there is
a consistent and enforced coding-style (irrespective of what the coding
style is) it makes it _significantly_ easier for collaborators and
maintainers to get on with the job in hand. It also, from a day-2-day
developer perspective, significantly reduces cognitive overhead re
reading code.
Finally this is a "trial balloon" PR, if this patch is accepted I will
likely do more work on testing and docs for the project.

- The new prettier config is a non-standard setup, but a close-match to
  how the code _currently_ looks.
- 120 col-width print width (instead of the better and more
  information-dense 88), this is so the diff after applying prettier to
  the code is less disruptive, whilst still showing the benefits of using
  a prettier.
- We use `tabWidth` setting of 4 as the code uses that more common
  setting instead of the more compact 2 spaces - note that 2 often leads
  to more readable and compact code.
- We enforce trailing commas, as that seems to be the norm in this
  code-base. We choose the `es5` standard here.
- We enforce tailing semi-colons (`semi`) as the majority of code used
  that flavour of `js`/`ts`.
- For now we only run on code and not json files.

This is designed such that `npm run pretty` re-formats the code for
development, and when integrated with ci/cd, `prettyCheck` will return
non-zero exit codes when formatting doesn't match the coding standards.

package-lock.json

@@ -58,6 +58,7 @@
                 "pouchdb-merge": "^9.0.0",
                 "pouchdb-replication": "^9.0.0",
                 "pouchdb-utils": "^9.0.0",
+                "prettier": "^3.3.3",
                 "svelte": "^4.2.19",
                 "svelte-preprocess": "^6.0.2",
                 "terser": "^5.31.6",
@@ -5690,6 +5691,22 @@
                 "node": ">= 0.8.0"
             }
         },
+        "node_modules/prettier": {
+            "version": "3.3.3",
+            "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.3.3.tgz",
+            "integrity": "sha512-i2tDNA0O5IrMO757lfrdQZCc2jPNDVntV0m/+4whiDfWaTKfMNgR7Qz0NAeGz/nRqF4m5/6CLzbP4/liHt12Ew==",
+            "dev": true,
+            "license": "MIT",
+            "bin": {
+                "prettier": "bin/prettier.cjs"
+            },
+            "engines": {
+                "node": ">=14"
+            },
+            "funding": {
+                "url": "https://github.com/prettier/prettier?sponsor=1"
+            }
+        },
         "node_modules/psl": {
             "version": "1.9.0",
             "resolved": "https://registry.npmjs.org/psl/-/psl-1.9.0.tgz",
@@ -10787,6 +10804,12 @@
             "integrity": "sha512-vkcDPrRZo1QZLbn5RLGPpg/WmIQ65qoWWhcGKf/b5eplkkarX0m9z8ppCat4mlOqUsWpyNuYgO3VRyrYHSzX5g==",
             "dev": true
         },
+        "prettier": {
+            "version": "3.3.3",
+            "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.3.3.tgz",
+            "integrity": "sha512-i2tDNA0O5IrMO757lfrdQZCc2jPNDVntV0m/+4whiDfWaTKfMNgR7Qz0NAeGz/nRqF4m5/6CLzbP4/liHt12Ew==",
+            "dev": true
+        },
         "psl": {
             "version": "1.9.0",
             "resolved": "https://registry.npmjs.org/psl/-/psl-1.9.0.tgz",