vscode-concourse: support both .yml and .yaml files

[bmalehorn]

Some people write .yml, some write .yaml. Both pipeline.yml and pipeline.yaml behave the same way, so highlight them both as Concourse pipelines & tasks.

Added because my organization uses pipeline.yaml consistently, so probably other people do too.

Reproduction

1. create a file ~/tmp/pipeline.yaml
2. run extension
3. open terminal
4. code ~/tmp/pipeline.yaml
5. look at the bottom to see if it says "Concourse Pipeline" or "YAML"

before	after

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[martinlippert]

Thanks a lot @bmalehorn for the contribution, much appreciated!!!

@kdvolder Please take a second look here and commit additional changes if you think we need anything else here.