Move idea to programming/configuration_files_proposal and flesh out

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+# Configuration files proposal
+
+No one will ever agree on the ideal configuration file format.
+Therefore, we should allow people to use their preferred configuration file format in all programs.
+
+## Rough idea
+
+Write libraries for as many languages as possible that read files like:
+
+```
+#? yaml
+my configuration:
+  option: value
+```
+
+```
+#? json
+{"my configuration": {"option": "value"}}
+```
+
+and produces a JSON string with the "parsed" input from the second line onwards.
+(Or optionally, a data structure appropriate for the language that you are using.)
+
+If your program uses this library to read configuration, then your users can write their configuration in their preferred language.
+
+## Challenges
+
+* How to prevent arbitrary code execution and other attacks?
+* How to support as many configuration languages as possible?
+
+## Possible solutions
+
+Use capability-based runtimes and embed capabilities in the first line of the configuration file:
+
+```
+                                                  # Allow the specific format parser to...
++file(./**)                                       # ... read files in the same directory as the configuration file and descendants.
++hostname                                         # ... access the hostname.
++load(https://example.com/file,sha256:...,/file)  # ... download a file from the Internet and expose it to the parser.
+...
+```
+
+Be able to refer to different parsers:
+
+```
+# Built-in parsers
+json
+yaml
+...
+
+# Parsers installed on the local system
+/path/to/my/parser
+
+# Download parsers on demand
+https://example.com/foo-parser
+```
+
+## Possible implementation
+
+Use WASM runtimes:
+
+* They already support capabilities.
+* Parsers can be distributed as WASM blobs.
+
+## Motivating examples
+
+### Declarative CI
+
+`.github/workflows/workflow.yaml`:
+
+```
+#? https://jsonnet.org/jsonnet.wasm +load(https://jsonnetci.com/rust.libsonnet,sha256:...,/rust.libsonnet)
+local rust = import '/rust.libsonnet';
+
+rust.github({
+  'supported-archs': ['x86_64-unknown-linux-gnu', ...],
+  'attach-binaries-to-releases': true,
+  'upload-to-docsrs': true,
+  ...
+})
+```
+
+but also `.gitlab-ci.yml`:
+
+```
+#? https://jsonnet.org/jsonnet.wasm +load(https://jsonnetci.com/rust.libsonnet,sha256:...,/rust.libsonnet)
+local rust = import '/rust.libsonnet';
+
+rust.gitlab(...)
+```