finch lsp: the language server, written in Finch

Hover, go to definition, completion (after a dot too: fields, methods, a module's names, an enum's variants,
array/text/map methods), signature help, the outline and errors while typing -- everything the C++ one did,
and the same tests/lsp_test.py passes against it.

It doesn't type-check by itself: for every change it runs  finch index  on the editor's text (--overlay says
which file the text stands in for) and reads what the compiler learned about every name, which is also what
the compiler's own error messages are made of. Half-typed code keeps using the last version that compiled.
src/docs.fch is the one description of the built-ins that hover and completion share.

tests/run.sh runs the LSP test too. 107 passed.

shell.nix

@@ -6,6 +6,7 @@ pkgs.mkShell {
     llvmPackages.clang
     llvmPackages.libclang   # reads C headers for `import "stdio.h"`
     pkg-config              # finds the right flags for `link "..."`
+    python3                 # tests/lsp_test.py
 
     # for the graphics examples (window, opengl, raylib, vulkan)
     glfw