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/server/scriptInfo.ts

@@ -167,7 +167,7 @@ namespace ts.server {
             const fileName = tempFileName || this.fileName;
             const getText = () => text === undefined ? (text = this.host.readFile(fileName) || "") : text;
             // Only non typescript files have size limitation
-            if (!hasTypeScriptFileExtension(this.fileName)) {
+            if (!hasTypescriptFileExtension(this.fileName)) {
                 const fileSize = this.host.getFileSize ? this.host.getFileSize(fileName) : getText().length;
                 if (fileSize > maxFileSize) {
                     Debug.assert(!!this.info.containingProjects.length);