Update texts for refactor: extract variable

[jenny-codes]

Motivation

Addressing the two comments (this and this) from @andyw8 in #491 that I didn't get to before merging.

Implementation

Two text/copy changes:

1. "Invalid selection for extract refactor" -> "Invalid selection for Extract Variable refactor"
2. "Refactor: Extract variable" -> "Refactor: Extract Variable"

Automated Tests

No changes

Manual Tests

No changes

[github-actions]

Benchmark results in seconds (slowest at top)

          textDocument/formatting average: 0.104504 std_dev: 0.027602
          textDocument/diagnostic average: 0.038526 std_dev: 0.001839
      textDocument/selectionRange average: 0.003804 std_dev: 0.000894
        textDocument/foldingRange average: 0.001613 std_dev: 7.0e-05
   textDocument/documentHighlight average: 0.001514 std_dev: 0.00016
 textDocument/semanticTokens/full average: 0.000829 std_dev: 0.000188
textDocument/semanticTokens/range average: 0.000626 std_dev: 4.9e-05
               textDocument/hover average: 0.0006 std_dev: 8.9e-05
        textDocument/documentLink average: 0.000538 std_dev: 4.4e-05
           textDocument/inlayHint average: 0.000416 std_dev: 4.0e-05
      textDocument/documentSymbol average: 0.000255 std_dev: 5.7e-05
    textDocument/onTypeFormatting average: 0.000111 std_dev: 8.4e-05
          textDocument/codeAction average: 3.1e-05 std_dev: 3.1e-05


================================================================================
Comparison with main branch:

 textDocument/semanticTokens/full unchanged
textDocument/semanticTokens/range unchanged
      textDocument/documentSymbol unchanged
        textDocument/foldingRange unchanged
          textDocument/formatting unchanged
          textDocument/diagnostic unchanged
        textDocument/documentLink unchanged
           textDocument/inlayHint unchanged
      textDocument/selectionRange unchanged
   textDocument/documentHighlight unchanged
               textDocument/hover unchanged
          textDocument/codeAction unchanged
    textDocument/onTypeFormatting unchanged