cmListFileLexer: Do not require files to be seekable

Sergiu Deitschrequested to merge
sergiud/cmake:sequential-bom into master

Read the BOM sequentially and store the read bytes for later use if these do not correspond to a BOM. This allows FIFO to be used as input, e.g., for piping input or Bash process substitution.

Closes: #25162 (closed)

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