Allow l.v. ties

[craigsapp]

Allow laissez vibrer ties.

Something like:

**kern
=1
!LO:T:lv
[2e
2r
=
*-

The default endpoint will be the next note/rest (2r in this case). Later, add some duration control for having the tie end somewhere else.

Here is the current rendering, where the tie is highlighted in red to indicate a syntax error (tie is not closed):

The l.v. parameter would suppress error coloring of incomplete ties.

[craigsapp]

Implemented with commit https://github.com/rism-digital/verovio/commit/639b98f89b3a39863496d2026a64971dccb8b26b

Example:

The first tie is highlighted in red because it is invalid (tie should be closed on a note of the same pitch). The second tie is valid because it is marked as a l.v. tie.

**kern
=1
[2e
2r
=2
!LO:T:lv
[2e
2r
=
*-

Click to view MEI conversion of above example

```xml