Add Sline language support

[shopline-developer]

Add Sline (.slt) markup language support. Sline is an HTML templating language used by the SHOPLINE e-commerce platform for storefront themes.

Description

Sline is a markup/templating language that extends HTML with template syntax ({{ }}, {{#if}}, filters like {{ value | filter() }}). It is used by merchants on the SHOPLINE platform to build and customize online store
themes.

• Grammar repository: https://github.com/shoplineos/sline-textmate
• Language homepage: https://developer.shopline.com/docs/sline/sline-overview

Checklist:

• I am adding a new language.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Written specifically for this PR
    • Sample license(s):
      • MIT (same as Linguist)
  • I have included a syntax highlighting grammar: https://github.com/shoplineos/sline-textmate
  • I have added a color
    • Hex value: #5F9AE0
    • Rationale: Blue tone reflecting the SHOPLINE brand identity
  • I have updated the heuristics to distinguish my language from others using the same extension.
    • .slt is not used by any other language in Linguist, so no heuristics are needed.

References:

Developer documentation: https://developer.shopline.com/docs/sline/sline-overview

VS Code extension: https://marketplace.visualstudio.com/items?itemName=shopline-developer.shopline-developer-plugin

TextMate grammar: https://github.com/shoplineos/sline-textmate

NPM package: https://www.npmjs.com/package/@shoplineos/sline-textmate

Maintained by: https://shopline.com

[lildude]

You've left off the first point from the PR template with the search result query that you are using to prove your language meets our usage requirements:

linguist/.github/PULL_REQUEST_TEMPLATE.md

Lines 22 to 25 in 537297c

	- [ ] The extension of the new language is used in hundreds of repositories on GitHub.com.
	- Search results for each extension:
	<!-- Replace FOOBAR with the new extension, and KEYWORDS with keywords unique to the language. Repeat for each extension added. -->
	- https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.FOOBAR+KEYWORDS

Please add and fill this in. If your language is a long way from meeting our usage requirements (see the end of the CONTRIBUTING.md file), I recommend you close this PR and submit a new one when your language meets our usage requirements.