Adding support for fish shell (CON-1840)

[noranraskin]

Description

Adding export.fish which does the same as export.sh just for fish-shell. The provided solution in #1206 didn't work for me as after running bash -c "source export.sh didn't set ESP_MATTER_PATH thus building didn't work

Related

Fixes #1206

Testing

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Tested both source export.fish and source ~/esp-matter/export.fish and compiled an example afterwards successfully.

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[CLAassistant]

All committers have signed the CLA.