ctags: filterSketchSource: enlarge buffer size to handle long lines

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Fixes https://github.com/arduino/ArduinoCore-zephyr/issues/140

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• [x] The PR has no duplicates (please search among the Pull Requests before creating one)
• [x] The PR follows our contributing guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] UPGRADING.md has been updated with a migration guide (for breaking changes)
• [ ] configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

When parsing the preprocessed file, only the lines relevant for the sketch are preserved. However, golang's scanner fails if a line is too long for its buffer. Overcome the issue by adding a 1MB buffer (extremely conservative) and a non-fatal error message in case it's still not enough.

What is the current behavior?

Compiling any sketch from https://github.com/arduino/ArduinoCore-zephyr fails to create prototypes due to analogPins macro, which expands into a ~90K characters line

What is the new behavior?

The prototypes are created correctly

Does this PR introduce a breaking change, and is titled accordingly?

No breaking change

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