Suppress dirty submodules in VSCode (#38)

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## Description
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When running in a dev container, certain submodules show changes in
VSCode.
GitHub Desktop does not show them.

This PR also adds missing Submodules to the devcontainer.json
git.ignoredRepositories.

## Motivation and Context
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Speculative fix. If there is a better way, it is welcome!

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## Types of changes
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- [ ] Improvement (non-breaking change that adds a new feature)
- [ ] Bug fix (fixes an issue)
- [ ] Breaking change (breaking change)
- [x] Config and build (change in the configuration and build system,
has no impact on code or features)

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- [ ] My code follows the code style of this project.
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- [ ] All new and existing tests passed.

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.devcontainer/devcontainer.json

@@ -16,8 +16,10 @@
             "settings": {
                 "git.ignoredRepositories": [
                     "libdragon",
+                    "src/libs/libspng",
                     "src/libs/mini.c",
-                    "src/libs/minimp3"
+                    "src/libs/minimp3",
+                    "src/libs/miniz.c"
                 ]
             }
         }

.gitmodules

@@ -2,15 +2,20 @@
 	path = libdragon
 	url = https://github.com/DragonMinded/libdragon
 	branch = unstable
+	ignore = dirty
 [submodule "src/libs/libspng"]
 	path = src/libs/libspng
 	url = https://github.com/randy408/libspng.git
+	ignore = dirty
 [submodule "src/libs/mini.c"]
 	path = src/libs/mini.c
 	url = https://github.com/univrsal/mini.c.git
+	ignore = dirty
 [submodule "src/libs/minimp3"]
 	path = src/libs/minimp3
 	url = https://github.com/lieff/minimp3.git
+	ignore = dirty
 [submodule "src/libs/miniz"]
 	path = src/libs/miniz
 	url = https://github.com/richgel999/miniz.git
+	ignore = dirty