Adding instructions, ESC and Ctrl-Z functionality and some bug fixes.

[lrosemberg]

Description

Here's what this PR does (video - 1 min):
https://share.cleanshot.com/FhgfqYfR

The following features have been added (all with an open issue):

• Add option to press ESC to clear last unsaved Polygon (Resolves Add option to press ESC to clear last unsaved Polygon #7)
• Add the Ctrl/Cmd+Z functionality (Resolves Add the Ctrl/Cmd+Z functionality. #9)
• Add some instructions to the webpage (Resolves Add some instructions to the webpage #8)

Bugfixes:

• The last line that connected to the mouse kept appearing when enter is pressed (Resolves Bugfix: the last line that connected to the mouse kept appearing when enter is pressed #6)

Additional:

• I simplified some logics, such as keyboard events

List any dependencies that are required for this change.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Docs

• Docs updated? What were the changes: Included the new functionalities

[lrosemberg]

Hi, can you check this please?

@capjamesg @yeldarby

[capjamesg]

This PR works as described in the PR description. Great work!