Add Skeleton component

[david0xd]

Add Skeleton component.

Related PR in extension: MetaMask/metamask-extension#29764

Code example

JSX code used for testing:

            <Container>
              <Box>
                <Text>Skeleton inside text component:</Text>
                <Text>
                  <Skeleton />
                </Text>
                <Text>Classic Skeleton inside a box:</Text>
                <Skeleton />
                <Text>Top level Skeleton: </Text>
                <Skeleton height={32} width="100%" />
                <Skeleton height={16} width="50%" borderRadius="none" />
                <Skeleton height={16} width="25%" borderRadius="medium" />
                <Skeleton height={32} width={32} borderRadius="full" />
                <Text>Skeleton inside Section: </Text>
                <Section>
                  <Skeleton height={32} width="100%" />
                  <Skeleton height={16} width="50%" borderRadius="none" />
                  <Skeleton height={16} width="25%" borderRadius="medium" />
                  <Skeleton height={32} width={32} borderRadius="full" />
                </Section>
                <Text>Skeleton inside Row: </Text>
                <Row label="Row">
                  <Skeleton height={22} width="30%" />
                </Row>
                <Row label="Row">
                  <Text>
                    <Skeleton height={22} width={40} />
                  </Text>
                </Row>
              </Box>
            </Container>

Screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (657cf53) to head (fd05758).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3024   +/-   ##
=======================================
  Coverage   94.76%   94.76%           
=======================================
  Files         499      500    +1     
  Lines       10906    10911    +5     
  Branches     1672     1672           
=======================================
+ Hits        10335    10340    +5     
  Misses        571      571           

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