Refactor run.sh: added custom build-plan file and build plan checks; improved messaging

[OnkelTem]

See: https://github.com/be5invis/Iosevka/issues/1340

This PR adds some messaging and checks.

From the very beginning of the run.sh script:

1) It prints which build-plans file it's gonna use 2) It prints which build plan it's gonna use 3) It doesn't dismiss the build plan anymore when there is no custom build-plains file. It's getting passed down to npm run build.

Sample session, without custom build-plans file and build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build
Custom build-plans file not found, using the default one
Using build plan: contents::iosevka
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

With custom build-plans file, but without a build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build
Found custom build-plans file: private-build-plans.toml
Using build plan: contents::iosevka-sans-q
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

With custom build-plans file and custom build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build ttf::iosevka-sans-q
Found custom build-plans file: private-build-plans.toml
Using build plan: ttf::iosevka-sans-q
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

[OnkelTem]

Wait, I found a mistake

[nobk]

iosevka-build can use like this, but you patch can not. iosevka-build ttf::my_font1 ttf::my_font2 ttf::my_font3 ttf::my_font4 ttf::my_font5