Rename JS-specific concepts (#26795)

* Rename JS concepts

1. Assignment declaration -- an assignment that is treated like a
declaration. Previously called [JS] special (assignment|declaration),
among other things.
2. Expando -- a value that can be used as a target in assignment
declarations. Currently, a class, function or empty object literal.
Functions are allowed in Typescript, too. Previously called a JS
container, JS initializer or expando object.
3. JavaScript -> Javascript. This is annoying to type, and looks like
'Java Script' in a camelCase world.

Everything is a pure rename as far as I know. The only test change is
the API baselines, which reflect the rename from SymbolFlags.JSContainer
to SymbolFlags.Assignment.

* Remove TODO

* Rename Javascript->JS

Note that this introduces a variable name collision in a couple of
places, which I resolved like this:

```ts
const isInJavascript = isInJSFile(node);
```

src/jsTyping/jsTyping.ts

@@ -122,7 +122,7 @@ namespace ts.JsTyping {
         // Only infer typings for .js and .jsx files
         fileNames = mapDefined(fileNames, fileName => {
             const path = normalizePath(fileName);
-            if (hasJavaScriptFileExtension(path)) {
+            if (hasJavascriptFileExtension(path)) {
                 return path;
             }
         });
@@ -218,7 +218,7 @@ namespace ts.JsTyping {
          */
         function getTypingNamesFromSourceFileNames(fileNames: string[]) {
             const fromFileNames = mapDefined(fileNames, j => {
-                if (!hasJavaScriptFileExtension(j)) return undefined;
+                if (!hasJavascriptFileExtension(j)) return undefined;
 
                 const inferredTypingName = removeFileExtension(getBaseFileName(j.toLowerCase()));
                 const cleanedTypingName = removeMinAndVersionNumbers(inferredTypingName);