docs: Document npm_old_version and npm_new_version environment variables (#9389)

Backport of #9385 to `release/v11`.

Co-authored-by: Oliver Byford <oliver.byford@digital.cabinet-office.gov.uk>

Author: github-actions[bot]

docs/lib/content/commands/npm-version.md

@@ -70,6 +70,8 @@ The exact order of execution is as follows:
 6. Run the `postversion` script.
    Use it to clean up the file system or automatically push the commit and/or tag.
 
+For the `preversion`, `version` and `postversion` scripts, npm also sets the [environment variables](/using-npm/scripts#environment) `npm_old_version` and `npm_new_version`.
+
 Take the following example:
 
 ```json

docs/lib/content/using-npm/scripts.md

@@ -290,6 +290,13 @@ For example, if you had `{"name":"foo", "version":"1.2.5"}` in your package.json
 
 See [`package.json`](/configuring-npm/package-json) for more on package configs.
 
+#### versioning variables
+
+For versioning scripts (`preversion`, `version`, `postversion`), npm sets these environment variables:
+
+* `npm_old_version` - The version before being bumped
+* `npm_new_version` – The version after being bumped
+
 #### current lifecycle event
 
 Lastly, the `npm_lifecycle_event` environment variable is set to whichever stage of the cycle is being executed.

workspaces/libnpmversion/README.md

@@ -86,6 +86,9 @@ The exact order of execution is as follows:
 6. Run the `postversion` script. Use it to clean up the file system or
    automatically push the commit and/or tag.
 
+For the `preversion`, `version` and `postversion` scripts, npm also sets the
+environment variables `npm_old_version` and `npm_new_version`.
+
 Take the following example:
 
 ```json