docs: use gjs as codefence lang

[IgnaceMaes]

Glimmer JS/TS syntax highlighting support was added to GitHub a couple of months ago. This PR uses that lang for codefences where <template> tag is being used.

[NullVoxPopuli]

gjs works:

const two = 2;

<template>
  {{two}}
</template>

gts:

const two = 2;

<template>
  {{two}}
</template>

glimmer-ts:

const two = 2;

<template>
  {{two}}
</template>

did we misconfigure something in linguist?

[IgnaceMaes]

It's because gts already existed prior to us adding GJS/GTS support.

In search you can see it detects it as Gerber image:

https://github.com/search?q=%28path%3A*.gts%29+AND+%28content%3A%3Ctemplate%3E+OR+content%3A%40glimmer+OR+content%3A%40ember%29+NOT+is%3Afork&type=code

Not sure if this can be changed, or if GitHub works with first come, first served.

[NullVoxPopuli]

hmm. that's silly