lsh: add Bash, Zsh, Python and JavaScript highlighting

[barkure]

This adds initial LSH syntax highlighting support for:

• Bash
• Zsh
• Python
• JavaScript

It also updates Markdown fenced code block routing so that:

• bash, sh, and shell use Bash highlighting
• zsh uses Zsh highlighting
• python / py use Python highlighting
• javascript / js / jsx / mjs / cjs use JavaScript highlighting

Scope

This is a lightweight LSH-only change with no new dependencies.

The changes are limited to syntax definitions and highlighting fixtures:

• new LSH definitions for the languages above
• Markdown fenced code block routing updates
• new / expanded highlighting test samples

Validation

• cargo test -q

Notes

The new rules are intentionally simple and follow the style of the existing LSH definitions.
They focus on the main token classes already used elsewhere in the project, such as:

• comments
• strings
• keywords
• language constants
• numeric literals
• common function-style identifiers

If you would prefer this to be discussed as a feature request first, I am happy to open an issue and rework the PR accordingly.

[barkure]

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