Add significand to BigFloat, fix #7207
Add tests for significand(::BigFloat)
fix #7203 and #7218

for Windows users who have git configured with \r\n line endings,
or modify the source in an editor that always uses \r\n
fix #7248: throw boundserror for invalid indices in ind2chr/chr2ind
add checkbounds(s::String, I)
REPL: Moving left and right was skipping newlines

Issue #7210 fixed movement across combining characters, such that moving left
and right by one character would skip all zero-width characters, but this also
inadvertently includes newlines. This made it impossible to navigate to the end
of a line that ended with a newline with the left and right arrows alone. And movement across multiple newlines would skip across them all at once.

This fixes that and adds simple regression tests.
Merge pull request #7211 from andrioni/add_significand

Add significand to BigFloat, fix #7207
Merge pull request #7220 from tkelman/patch-4

fix #7203 and #7218
Merge pull request #7253 from mbauman/char-move-skips-newlines

REPL: Moving left and right was skipping newlines
Merge pull request #7249 from stevengj/ind2chr_bounds

fix #7248: throw boundserror for invalid indices in ind2chr/chr2ind
isvalid() chr2ind() ind2chr() for SubString of DirectIndexString