Detect file encoding from byte order mark

[dronov-dmitry]

Summary

Fixes opening files that are not UTF-8: the editor used the configured default encoding (UTF-8) for every file, so e.g. a UTF-16LE file was shown as garbage. Now the file encoding is detected from its byte order mark (BOM) and the file is opened (and saved) in the correct encoding.

How it works

• New utils/FileEncoding detects the BOM:
  • UTF-32 LE/BE, UTF-8, UTF-16 BE/LE (in order of signature length so the 4-byte UTF-32 LE signature is not misread as UTF-16 LE).
• On open (openNamedFileDirect, openNamedFileLegacyDirect, applyExternalDocument) the detected encoding is used and the BOM is stripped from the text.
• On save the document is written back in its original encoding with the BOM restored, so the file on disk stays unchanged in format.
• When no BOM is present, the configured default encoding is used as before.

Verification

• New unit tests FileEncodingTest (10 cases: detection, BOM stripping, encode round-trip) — all pass.
• ./gradlew compileDebugJavaWithJavac — OK
• ./gradlew testDebugUnitTest — OK
• ./gradlew lint — no errors
• Verified against a real UTF-16LE (BOM) Cyrillic report file.