Build/Test Tools: Add .nvmrc files to default themes.

Today there are 5 default themes that have `package.json` files:
- Twenty Nineteen
- Twenty Twenty
- Twenty Twenty-One
- Twenty Twenty-Two
- Twenty Twenty-Five

The `.nvmrc` file at the root of the repository will be used when modifying these themes in the context of a `wordpress-develop` checkout. But these themes exist outside of `wordpress-develop` more often than not. And there are times where the version of Node.js used by Core is not the same as one or more of the themes (this is currently the case with Twenty Nineteen).

This adds an `.nvmrc` file to each of the 5 themes that rely on Node.js. The theme testing GitHub Actions workflow is also updated to use each theme’s individual `.nvmrc` files to determine which version of Node.js to use instead of always defaulting to the version used for Core itself.

Fixes #66041.

git-svn-id: https://develop.svn.wordpress.org/trunk@63450 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

src/wp-content/themes/twentynineteen/.nvmrc

@@ -0,0 +1 @@
+20

src/wp-content/themes/twentytwenty/.nvmrc

@@ -0,0 +1 @@
+16

src/wp-content/themes/twentytwentyfive/.nvmrc

@@ -0,0 +1 @@
+20

src/wp-content/themes/twentytwentyone/.nvmrc

@@ -0,0 +1 @@
+20

src/wp-content/themes/twentytwentytwo/.nvmrc

@@ -0,0 +1 @@
+20