Add ts and tsx file associations to defaults

[Afnan Enayet]

As noted by another user, we should add the typescript grammar file associations to the defaults in diffsitter.

          Running `diffsitter` `0.8.1` on macOS via homebrew:

⇒ diffsitter --version
diffsitter 0.8.1

It lists support for typescript / tsx:

⇒ diffsitter list
This program was compiled with support for:
- bash
- c_sharp
- cpp
- css
- go
- hcl
- java
- json
- ocaml
- php
- python
- ruby
- rust
- tsx
- typescript

Yet by default, it will fail to run against a JavaScript file:

⇒ git difftool --tool diffsitter HEAD~1 HEAD -- unpacked/_next/static/\[buildHash\]/_buildManifest.js
Error: Unsupported file type with no fallback command specified.

Until a file-association override is added to the config (${XDG_HOME:-$HOME}/.config/diffsitter/config.json5):

// ..snip..
  "grammar": {
    "dylib-overrides": null,
    "file-associations": {
      "js": "typescript",
      "jsx": "tsx"
    },
  },
// ..snip..

This would seem like a useful thing to be included in the default config that diffsitter uses:

⇒ diffsitter dump-default-config
{
  "file-associations": null,
  "formatting": {
    "default": "unified",
    "unified": {
      "addition": {
        "highlight": null,
        "regular-foreground": "green",
        "emphasized-foreground": "green",
        "bold": true,
        "underline": false,
        "prefix": "+ "
      },
      "deletion": {
        "highlight": null,
        "regular-foreground": "red",
        "emphasized-foreground": "red",
        "bold": true,
        "underline": false,
        "prefix": "- "
      }
    },
    "json": {
      "pretty_print": false
    },
    "custom": {}
  },
  "grammar": {
    "dylib-overrides": null,
    "file-associations": null
  },
  "input-processing": {
    "split-graphemes": true,
    "exclude-kinds": null,
    "include-kinds": null
  },
  "fallback-cmd": null
}

I also noted that modifying that default config to add the file-associations to the root key didn't seem to work; it only seemed to work when I added them to the grammar version of file-associations.

Originally posted by @0xdevalias in #152 (comment)