This mimics VSCode's `files.readonlyExclude` setting, to allow setting
specific path matches as readonly locations like lockfiles and generated
sources etc.

Also renders a lock icon to the right side of the path names for
readonly files now.
This does a couple more things for completion sake:
- Tabs of readonly buffers now render a file lock icon
- Multibuffer buffer headers now render a file lock icon if the excerpts
buffer is readonly
- ReadWrite multibuffers now no longer allow edits to read only buffers
contained within

Release Notes:

- Added `read_only_files` setting to allow specifying glob patterns of
files that should not be editable by default

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>