Add current-pair and file-text snippet vars

[PEZ]

What has changed?

Adding current-pair and file-text repl snippet vars.

• Fixes Add custom snippet $current-pair variable  #1943
• Fixes Add custom repl command snippet var $file-text #1944

My Calva PR Checklist

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Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik